diff --git a/.ciagent/CA_MIGRATION_SPEC_v0.9.md b/.ciagent/CA_MIGRATION_SPEC_v0.9.md
new file mode 100644
index 0000000..27bc754
--- /dev/null
+++ b/.ciagent/CA_MIGRATION_SPEC_v0.9.md
@@ -0,0 +1,109 @@
+# CA Migration Spec — v0.8 Internal CA → v0.9 step-ca (grill C-07)
+
+**Status**: spec (must be implemented in v0.10-P14a, REQ-066)
+**Gate**: C-07 — blocks v0.10-P14a until this spec is reviewed and a dry-run passes on a test cluster
+
+## Problem
+
+The v0.8 internal Go CA (`internal/security/ca.go`) issues RSA-3072 CA
+certs (10-year validity) and ECDSA P-256 server certs (90-day). The CA
+material lives at `~/.orca/ca.crt` and `~/.orca/ca.key` (flat layout, D-011).
+The v0.9 re-architecture reverses AD-010 and replaces the internal CA with
+step-ca (D-101, REQ-076). Existing v0.8 deployments have an internal CA
+root + issued server certs that must be migrated without invalidating
+trust across the cluster.
+
+## Migration options (decision required before v0.10-P14a implementation)
+
+### Option A — Preserve trust root (RECOMMENDED)
+
+Import the existing `ca.key` into step-ca as the root CA key. The cluster's
+trust fingerprint stays unchanged; existing server certs continue to
+validate until their natural expiry; new SVIDs are minted by step-ca using
+the same root.
+
+```bash
+orca upgrade --to-v1.0 --import-ca
+# reads ~/.orca/ca.key → step ca init --deployment-type standalone \
+#   --remote-management --key $(cat ~/.orca/ca.key)
+# issues new SVIDs from step-ca for all existing workloads
+```
+
+**Pros**: zero trust breakage; existing server certs keep working; minimal
+operator disruption.
+**Cons**: requires step-ca to accept an imported RSA-3072 key (step-ca
+supports imported keys via `--key` flag; verify in the spike).
+**Post-migration**: old `internal/security/ca.go` and `csr.go` are deleted
+(v0.10-P14); the `cert_repo` SQLite table (0004) is dropped (step-ca
+manages cert state).
+
+### Option B — Forced re-bootstrap
+
+Document that v0.8 certs are invalidated; every cluster re-bootstraps under
+step-ca with a new root. Existing workloads are re-enrolled.
+
+**Pros**: clean slate; no legacy RSA root.
+**Cons**: trust breakage — every peer's `known_hosts` + CA cert must be
+rotated; running workloads lose mTLS until re-enrolled; higher operator
+disruption.
+**Use case**: only if Option A is technically infeasible (step-ca rejects
+the v0.8 key format).
+
+## Pre-flight checks (must pass before migration)
+
+1. `orca doctor` reports zero FAILs on the v0.8 cluster
+2. All peers reachable via SSH
+3. No in-flight transactions (the migration is stop-the-world for the CA)
+4. Snapshot taken (`orca backup --include-master-key`)
+5. step-ca installed on the lead via `apt-get install step-ca`
+6. `step ca init` dry-run succeeds with the imported key
+
+## Migration steps (Option A)
+
+1. SSH to the lead; install step-ca via apt
+2. Run `step ca init --deployment-type standalone --remote-management \
+     --key <v0.8-ca-key-path> --provisioner orca-admin`
+3. Move the root cert: `cp ~/.orca/ca.crt $ORCA_HOME/cluster/ca.crt`
+4. Issue new SVIDs for every registered workload (via `step ca token` +
+   `step ca certificate` — the CLI mints the provisioner token using
+   `cluster/master.key`-derived material)
+5. Deploy the new SVIDs to peers via SSH-push (the v0.9 SSH-push transport)
+6. Verify: `orca doctor` reports zero FAILs; CA fingerprint unchanged;
+   all workload SVIDs valid
+7. Archive the old `internal/security/ca.go`/`csr.go` and `cert_repo` table
+
+## Rollback
+
+If any post-migration invariant fails:
+1. Restore the v0.8 snapshot via `orca upgrade --rollback <tarball>`
+2. Restart the v0.8 orca daemon on the lead
+3. Verify `orca doctor` passes on the v0.8 cluster
+
+The v0.8 internal CA remains functional during the dual-write window
+(REQ-090); step-ca is additive until the migration completes.
+
+## Post-migration invariants (must all pass)
+
+- CA fingerprint unchanged (Option A)
+- Node count unchanged
+- Workload count unchanged
+- All SVIDs valid (mTLS handshake succeeds lead↔every peer)
+- `orca doctor` zero FAILs
+- No `internal/security/ca.go` or `cert_repo` references remain in code
+
+## Decision required
+
+This spec is gated by C-07. The decision (Option A vs B) must be made
+before v0.10-P14a implementation. Default: Option A (preserve trust root)
+unless the step-ca imported-key spike fails.
+
+## Spike (must run before v0.10-P14a)
+
+Run on a test cluster:
+1. Install step-ca on a clean Linux host
+2. Generate a v0.8-style RSA-3072 CA key via the v0.8 `internal/security` package
+3. Run `step ca init --key <v8-key>` and verify step-ca accepts it
+4. Mint a test SVID via `step ca token` + `step ca certificate`
+5. Verify the SVID validates against the imported root
+
+If the spike fails, fall back to Option B (forced re-bootstrap) and document.
\ No newline at end of file
diff --git a/internal/certpaths/certpaths.go b/internal/certpaths/certpaths.go
index d009b94..d3ebec5 100644
--- a/internal/certpaths/certpaths.go
+++ b/internal/certpaths/certpaths.go
@@ -1,64 +1,73 @@
-// Package certpaths centralizes the on-disk locations of the CA and
-// server cert/key files. The CLI layer, the security layer, and the
-// doctor layer all need to agree on these paths, so they're factored
-// into their own package to avoid import cycles (cli <-> doctor).
+// Package certpaths is the v0.8 path shim. It returns v0.8 flat-layout
+// paths for backward compatibility during the v0.9 dual-write window
+// (REQ-090). The v0.9 paths package (internal/paths) returns the new
+// multi-namespace layout (R-002).
+//
+// certpaths will be deleted after the v0.10-P14 migration. New code
+// should use internal/paths, NOT certpaths.
+//
+// Migration notes (per v0.10-P14):
+//   - CA cert/key, server cert/key, SSH key/pub, known_hosts currently
+//     live at the flat Root() location. The v0.9 internal/paths package
+//     returns the new ClusterDir()/... locations; certpaths keeps the
+//     v0.8 flat locations until the CA migration moves them.
+//   - DBPath keeps returning Root()/orca.db (v0.8 location). The new
+//     paths.NSDb("_defaults") returns Root()/_defaults/db/orca.db; the DB
+//     moves in v0.10-P14.
 package certpaths
 
 import (
 	"os"
 	"path/filepath"
+
+	"git.cloudinit.dev/coreci/orca/internal/paths"
 )
 
-const (
-	defaultCADir   = ".orca"
-	caCertFilename = "ca.crt"
-	caKeyFilename  = "ca.key"
-)
+// Dir returns the v0.8 flat root directory. Delegates to paths.Root()
+// (which honors $ORCA_HOME, else ~/.orca). v0.8 callers expect the CA
+// and DB to live directly under this directory; that does not change
+// until the v0.10-P14 migration.
+func Dir() string { return paths.Root() }
 
-// Dir returns the directory the local CA lives in. Honors $ORCA_HOME
-// for testability; otherwise defaults to ~/.orca.
-func Dir() string {
-	if p := os.Getenv("ORCA_HOME"); p != "" {
-		return p
-	}
-	home, _ := os.UserHomeDir()
-	return filepath.Join(home, defaultCADir)
-}
+// CACertPath returns the v0.8 CA cert path: Dir()/ca.crt.
+// The v0.9 location is paths.CACertPath() = ClusterDir()/ca.crt; certpaths
+// keeps the v0.8 flat location until the CA migration in v0.10-P14.
+func CACertPath() string { return filepath.Join(paths.Root(), "ca.crt") }
 
-// CACertPath returns the path to ca.crt.
-func CACertPath() string { return filepath.Join(Dir(), caCertFilename) }
+// CAKeyPath returns the v0.8 CA key path: Dir()/ca.key.
+// See CACertPath for migration notes.
+func CAKeyPath() string { return filepath.Join(paths.Root(), "ca.key") }
 
-// CAKeyPath returns the path to ca.key.
-func CAKeyPath() string { return filepath.Join(Dir(), caKeyFilename) }
+// ServerCertPath returns the v0.8 server cert path: Dir()/server.crt.
+// See CACertPath for migration notes.
+func ServerCertPath() string { return filepath.Join(paths.Root(), "server.crt") }
 
-// ServerCertPath returns the path to server.crt.
-func ServerCertPath() string { return filepath.Join(Dir(), "server.crt") }
-
-// ServerKeyPath returns the path to server.key.
-func ServerKeyPath() string { return filepath.Join(Dir(), "server.key") }
+// ServerKeyPath returns the v0.8 server key path: Dir()/server.key.
+// See CACertPath for migration notes.
+func ServerKeyPath() string { return filepath.Join(paths.Root(), "server.key") }
 
 // DBPath returns the path to the orca SQLite database. Honors $ORCA_DB
-// for testability and explicit override; otherwise defaults to
-// ~/.orca/orca.db under the same Dir() as the cert files.
+// for testability and explicit override; otherwise defaults to the v0.8
+// flat location Dir()/orca.db. The v0.9 location is
+// paths.NSDb(paths.DefaultNamespace()) = Root()/_defaults/db/orca.db;
+// certpaths keeps the v0.8 flat location until the DB move in v0.10-P14.
 func DBPath() string {
 	if p := os.Getenv("ORCA_DB"); p != "" {
 		return p
 	}
-	return filepath.Join(Dir(), "orca.db")
+	return filepath.Join(paths.Root(), "orca.db")
 }
 
-// SSHKeyPath returns the path to the orca SSH private key (Ed25519,
-// D-037). Used by `orca node join --type proxmox` to authenticate
-// to remote Proxmox hosts after the initial password-based bootstrap.
-// File mode 0600 (enforced by security.WriteKey).
-func SSHKeyPath() string { return filepath.Join(Dir(), "orca_ssh_key") }
+// SSHKeyPath returns the v0.8 SSH private key path: Dir()/orca_ssh_key.
+// The v0.9 location is paths.SSHKeyPath() = ClusterDir()/orca_ssh_key;
+// certpaths keeps the v0.8 flat location until the migration.
+func SSHKeyPath() string { return filepath.Join(paths.Root(), "orca_ssh_key") }
 
-// SSHPubPath returns the path to the orca SSH public key (authorized_keys
-// format). Deployed to remote Proxmox hosts during `orca node join`.
-// File mode 0644 (enforced by security.WriteCert).
-func SSHPubPath() string { return filepath.Join(Dir(), "orca_ssh_key.pub") }
+// SSHPubPath returns the v0.8 SSH public key path: Dir()/orca_ssh_key.pub.
+// See SSHKeyPath for migration notes.
+func SSHPubPath() string { return filepath.Join(paths.Root(), "orca_ssh_key.pub") }
 
-// KnownHostsPath returns the path to the SSH known_hosts file used for
-// TOFU host-key pinning (D-035). Captured on first connect, verified
-// on all subsequent connects via golang.org/x/crypto/ssh/knownhosts.
-func KnownHostsPath() string { return filepath.Join(Dir(), "known_hosts") }
+// KnownHostsPath returns the v0.8 known_hosts path: Dir()/known_hosts.
+// The v0.9 location is paths.KnownHostsPath() = ClusterDir()/known_hosts;
+// certpaths keeps the v0.8 flat location until the migration.
+func KnownHostsPath() string { return filepath.Join(paths.Root(), "known_hosts") }
diff --git a/internal/certpaths/certpaths_test.go b/internal/certpaths/certpaths_test.go
index ced6761..ca84aa1 100644
--- a/internal/certpaths/certpaths_test.go
+++ b/internal/certpaths/certpaths_test.go
@@ -3,15 +3,17 @@ package certpaths
 import (
 	"os"
 	"path/filepath"
-	"runtime"
 	"strings"
 	"testing"
+
+	"git.cloudinit.dev/coreci/orca/internal/paths"
 )
 
+const defaultHomeSubdir = ".orca"
+
 func TestPaths_HonorORCAHOME(t *testing.T) {
 	dir := t.TempDir()
 	t.Setenv("ORCA_HOME", dir)
-	// Ensure ORCA_DB doesn't leak from the environment / prior tests.
 	t.Setenv("ORCA_DB", "")
 
 	cases := []struct {
@@ -36,17 +38,26 @@ func TestPaths_HonorORCAHOME(t *testing.T) {
 		})
 	}
 
-	// DBPath defaults to $ORCA_HOME/orca.db.
 	if got, want := DBPath(), filepath.Join(dir, "orca.db"); got != want {
 		t.Errorf("DBPath = %q, want %q", got, want)
 	}
 
-	// Dir() returns ORCA_HOME verbatim.
 	if got, want := Dir(), dir; got != want {
 		t.Errorf("Dir = %q, want %q", got, want)
 	}
 }
 
+func TestShim_DelegatesDirToPaths(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("ORCA_HOME", dir)
+	if got, want := Dir(), paths.Root(); got != want {
+		t.Errorf("Dir() = %q, paths.Root() = %q (shim must delegate)", got, want)
+	}
+	if got, want := Dir(), dir; got != want {
+		t.Errorf("Dir() = %q, want %q", got, want)
+	}
+}
+
 func TestDBPath_OrcaDBOverride(t *testing.T) {
 	home := t.TempDir()
 	t.Setenv("ORCA_HOME", home)
@@ -70,19 +81,14 @@ func TestDBPath_OrcaDBEmptyStringFallsBackToHome(t *testing.T) {
 }
 
 func TestDir_DefaultHomeFallback(t *testing.T) {
-	// Unset ORCA_HOME so Dir() falls back to ~/.orca.
-	// We can't reliably mutate the real HOME in a portable way, so just
-	// assert that the returned path ends with the default subdir on the
-	// current OS and is absolute.
 	os.Unsetenv("ORCA_HOME")
-	// Also clear ORCA_DB so DBPath's fallback to Dir() is exercised.
 	os.Unsetenv("ORCA_DB")
 
 	home, err := os.UserHomeDir()
 	if err != nil {
 		t.Skipf("os.UserHomeDir: %v (cannot verify default fallback)", err)
 	}
-	want := filepath.Join(home, defaultCADir)
+	want := filepath.Join(home, defaultHomeSubdir)
 	if got := Dir(); got != want {
 		t.Errorf("Dir() default = %q, want %q", got, want)
 	}
@@ -92,25 +98,22 @@ func TestDir_DefaultHomeFallback(t *testing.T) {
 }
 
 func TestDir_ORCAHOMEEmptyFallsBack(t *testing.T) {
-	// Empty string ORCA_HOME is treated as unset → ~/.orca fallback.
 	t.Setenv("ORCA_HOME", "")
 	home, err := os.UserHomeDir()
 	if err != nil {
 		t.Skipf("os.UserHomeDir: %v", err)
 	}
-	want := filepath.Join(home, defaultCADir)
+	want := filepath.Join(home, defaultHomeSubdir)
 	if got := Dir(); got != want {
 		t.Errorf("Dir() with empty ORCA_HOME = %q, want %q", got, want)
 	}
 }
 
 func TestDir_ORCAHOMERelativePath(t *testing.T) {
-	// A relative ORCA_HOME is honored verbatim (no cleaning/absolutizing).
 	t.Setenv("ORCA_HOME", "relative/orca/home")
 	if got, want := Dir(), "relative/orca/home"; got != want {
 		t.Errorf("Dir() relative = %q, want %q", got, want)
 	}
-	// CACertPath joins the relative dir with ca.crt using filepath.Join.
 	if got, want := CACertPath(), filepath.Join("relative/orca/home", "ca.crt"); got != want {
 		t.Errorf("CACertPath relative = %q, want %q", got, want)
 	}
@@ -121,7 +124,6 @@ func TestAllPaths_AreConsistentWithDir(t *testing.T) {
 	t.Setenv("ORCA_HOME", dir)
 	t.Setenv("ORCA_DB", "")
 
-	// Every *Path() must live under Dir() except DBPath which also does.
 	base := Dir()
 	for _, p := range []string{
 		CACertPath(), CAKeyPath(),
@@ -135,6 +137,38 @@ func TestAllPaths_AreConsistentWithDir(t *testing.T) {
 	}
 }
 
+func TestShim_ReturnsV08FlatPaths(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("ORCA_HOME", dir)
+	t.Setenv("ORCA_DB", "")
+
+	root := paths.Root()
+	if got, want := CACertPath(), filepath.Join(root, "ca.crt"); got != want {
+		t.Errorf("CACertPath = %q, want v0.8 flat %q", got, want)
+	}
+	if got, want := CAKeyPath(), filepath.Join(root, "ca.key"); got != want {
+		t.Errorf("CAKeyPath = %q, want v0.8 flat %q", got, want)
+	}
+	if got, want := ServerCertPath(), filepath.Join(root, "server.crt"); got != want {
+		t.Errorf("ServerCertPath = %q, want v0.8 flat %q", got, want)
+	}
+	if got, want := ServerKeyPath(), filepath.Join(root, "server.key"); got != want {
+		t.Errorf("ServerKeyPath = %q, want v0.8 flat %q", got, want)
+	}
+	if got, want := SSHKeyPath(), filepath.Join(root, "orca_ssh_key"); got != want {
+		t.Errorf("SSHKeyPath = %q, want v0.8 flat %q", got, want)
+	}
+	if got, want := SSHPubPath(), filepath.Join(root, "orca_ssh_key.pub"); got != want {
+		t.Errorf("SSHPubPath = %q, want v0.8 flat %q", got, want)
+	}
+	if got, want := KnownHostsPath(), filepath.Join(root, "known_hosts"); got != want {
+		t.Errorf("KnownHostsPath = %q, want v0.8 flat %q", got, want)
+	}
+	if got, want := DBPath(), filepath.Join(root, "orca.db"); got != want {
+		t.Errorf("DBPath = %q, want v0.8 flat %q", got, want)
+	}
+}
+
 func TestSSHPaths_Filenames(t *testing.T) {
 	dir := t.TempDir()
 	t.Setenv("ORCA_HOME", dir)
@@ -148,10 +182,3 @@ func TestSSHPaths_Filenames(t *testing.T) {
 		t.Errorf("KnownHostsPath base = %q, want %q", got, want)
 	}
 }
-
-func init() {
-	// On Windows the default home subdir is still ".orca"; the test for
-	// default fallback uses os.UserHomeDir which is platform-aware. This
-	// guard keeps the suite from running a meaningless check on plan9.
-	_ = runtime.GOOS
-}
diff --git a/internal/config/config.go b/internal/config/config.go
index b274855..6b52049 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -3,6 +3,7 @@ package config
 import (
 	"fmt"
 	"os"
+	"strings"
 
 	"github.com/hashicorp/hcl/v2/hclsimple"
 )
@@ -33,22 +34,82 @@ type Flags struct {
 
 type Environ map[string]string
 
+// Load is the config dispatcher (R-014). It tries each path in order,
+// skipping missing files, and dispatches to the appropriate loader
+// based on file extension: .hcl → LoadHCL (legacy, R-013),
+// .md → LoadMarkdown (new Markdown-frontmatter loader), and
+// .yaml/.yml → LoadMarkdown with an empty body. The first successfully
+// decoded file wins. If no path exists or decodes, a zero Config is
+// returned.
+//
+// The signature is preserved from the v0.8 single-loader API so
+// internal/cli/root.go requires no changes yet.
 func Load(paths ...string) (*Config, error) {
 	for _, p := range paths {
 		if _, err := os.Stat(p); err != nil {
 			continue
 		}
+		cfg, err := loadByExtension(p)
+		if err != nil {
+			return nil, err
+		}
+		return cfg, nil
+	}
+	return &Config{}, nil
+}
+
+func loadByExtension(p string) (*Config, error) {
+	ext := strings.ToLower(filepathExt(p))
+	switch ext {
+	case ".hcl":
+		return LoadHCL(p)
+	case ".md", ".markdown":
+		return LoadMarkdown(p)
+	case ".yaml", ".yml":
+		return LoadMarkdownYAML(p)
+	default:
+		// Unknown extension: hclsimple.Decode rejects non-.hcl
+		// suffixes, so for backward compat with the v0.8 single-loader
+		// behavior (which assumed HCL), decode the file content as HCL
+		// against a synthesized .hcl path.
 		data, err := os.ReadFile(p)
 		if err != nil {
 			return nil, fmt.Errorf("read config %s: %w", p, err)
 		}
 		var cfg Config
-		if err := hclsimple.Decode(p, data, nil, &cfg); err != nil {
+		if err := hclsimple.Decode(p+".hcl", data, nil, &cfg); err != nil {
 			return nil, fmt.Errorf("decode config %s: %w", p, err)
 		}
 		return &cfg, nil
 	}
-	return &Config{}, nil
+}
+
+// filepathExt is a thin wrapper around filepath.Ext to keep the import
+// localized to the dispatcher. Returns the extension including the dot,
+// lowercased by the caller.
+func filepathExt(p string) string {
+	i := strings.LastIndex(p, ".")
+	if i < 0 {
+		return ""
+	}
+	return p[i:]
+}
+
+// LoadHCL decodes a legacy HCL config file (R-013).
+//
+// Deprecated: use LoadMarkdown or the dispatcher. HCL is legacy per
+// R-013. Retained for the v0.9 dual-write window (REQ-090); new
+// deployments should author config.md with YAML frontmatter.
+func LoadHCL(path string) (*Config, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("read config %s: %w", path, err)
+	}
+	var cfg Config
+	if err := hclsimple.Decode(path, data, nil, &cfg); err != nil {
+		return nil, fmt.Errorf("decode config %s: %w", path, err)
+	}
+	return &cfg, nil
 }
 
 func (c *Config) MergeOverrides(flags Flags, env Environ) *Config {
diff --git a/internal/config/dispatch_test.go b/internal/config/dispatch_test.go
new file mode 100644
index 0000000..9e117ed
--- /dev/null
+++ b/internal/config/dispatch_test.go
@@ -0,0 +1,111 @@
+package config
+
+import (
+	"path/filepath"
+	"testing"
+)
+
+func TestLoad_DispatchHCL(t *testing.T) {
+	p := writeTestFile(t, t.TempDir(), "config.hcl", exampleHCL)
+	cfg, err := Load(p)
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if cfg.DBPath != "/tmp/orca/test.db" {
+		t.Errorf("DBPath=%q", cfg.DBPath)
+	}
+	if cfg.NodeCapacity == nil || cfg.NodeCapacity.CPU != 4 {
+		t.Errorf("NodeCapacity=%+v", cfg.NodeCapacity)
+	}
+}
+
+func TestLoad_DispatchMarkdown(t *testing.T) {
+	p := writeTestFile(t, t.TempDir(), "config.md", exampleMarkdown)
+	cfg, err := Load(p)
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if cfg.DBPath != "/tmp/orca/test.db" {
+		t.Errorf("DBPath=%q", cfg.DBPath)
+	}
+	if cfg.ListenAddr != "127.0.0.1:9999" {
+		t.Errorf("ListenAddr=%q", cfg.ListenAddr)
+	}
+	if cfg.NodeCapacity == nil || cfg.NodeCapacity.CPU != 4 {
+		t.Errorf("NodeCapacity=%+v", cfg.NodeCapacity)
+	}
+}
+
+func TestLoad_DispatchYAML(t *testing.T) {
+	body := "listen_addr: 0.0.0.0:5555\ndb_path: /bare.db\nnode_capacity:\n  cpu: 2\n  memory_mb: 4096\n"
+	p := writeTestFile(t, t.TempDir(), "config.yaml", body)
+	cfg, err := Load(p)
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if cfg.ListenAddr != "0.0.0.0:5555" {
+		t.Errorf("ListenAddr=%q", cfg.ListenAddr)
+	}
+	if cfg.DBPath != "/bare.db" {
+		t.Errorf("DBPath=%q", cfg.DBPath)
+	}
+	if cfg.NodeCapacity == nil || cfg.NodeCapacity.CPU != 2 {
+		t.Errorf("NodeCapacity=%+v", cfg.NodeCapacity)
+	}
+}
+
+func TestLoad_DispatchYML(t *testing.T) {
+	body := "listen_addr: 1.2.3.4:9\n"
+	p := writeTestFile(t, t.TempDir(), "config.yml", body)
+	cfg, err := Load(p)
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if cfg.ListenAddr != "1.2.3.4:9" {
+		t.Errorf("ListenAddr=%q", cfg.ListenAddr)
+	}
+}
+
+func TestLoad_DispatchFirstExistingWins(t *testing.T) {
+	dir := t.TempDir()
+	missing := filepath.Join(dir, "missing.md")
+	existing := writeTestFile(t, dir, "real.hcl", exampleHCL)
+	cfg, err := Load(missing, existing)
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if cfg.DBPath != "/tmp/orca/test.db" {
+		t.Errorf("DBPath=%q", cfg.DBPath)
+	}
+}
+
+func TestLoad_DispatchMissingReturnsZero(t *testing.T) {
+	cfg, err := Load(filepath.Join(t.TempDir(), "nope.md"))
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if cfg == nil {
+		t.Fatal("nil config")
+	}
+	if cfg.DBPath != "" || cfg.ListenAddr != "" || cfg.NodeCapacity != nil {
+		t.Errorf("expected zero config, got %+v", cfg)
+	}
+}
+
+func TestLoad_DispatchHCLMalformed(t *testing.T) {
+	p := writeTestFile(t, t.TempDir(), "bad.hcl", "db_path = ")
+	if _, err := Load(p); err == nil {
+		t.Fatal("expected error for malformed HCL")
+	}
+}
+
+func TestLoad_DispatchUnknownExtFallsBackToHCL(t *testing.T) {
+	p := writeTestFile(t, t.TempDir(), "config.unknown", exampleHCL)
+	cfg, err := Load(p)
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if cfg.DBPath != "/tmp/orca/test.db" {
+		t.Errorf("DBPath=%q", cfg.DBPath)
+	}
+}
diff --git a/internal/config/markdown.go b/internal/config/markdown.go
new file mode 100644
index 0000000..540670c
--- /dev/null
+++ b/internal/config/markdown.go
@@ -0,0 +1,220 @@
+package config
+
+import (
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+)
+
+// LoadMarkdown decodes a Markdown config file with YAML frontmatter
+// (R-014). The file format is:
+//
+//	---
+//	listen_addr: 127.0.0.1:9999
+//	node_capacity:
+//	  cpu: 4
+//	  memory_mb: 8192
+//	db_path: /tmp/orca/test.db
+//	---
+//
+//	body prose (ignored)
+//
+// The frontmatter parser is a minimal hand-rolled key:value parser
+// (no new dependencies; gopkg.in/yaml.v3 is not in go.mod). It supports
+// flat scalar keys and one level of nested mapping (for node_capacity).
+// The Markdown body after the closing "---" is ignored.
+//
+// The returned *Config is the same struct the HCL loader produces, so
+// downstream consumers are unchanged.
+func LoadMarkdown(path string) (*Config, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("read config %s: %w", path, err)
+	}
+	return parseFrontmatter(string(data), path)
+}
+
+// LoadMarkdownYAML decodes a bare YAML file (no Markdown body) using the
+// same minimal frontmatter parser. .yaml/.yml files are routed here by
+// the dispatcher.
+func LoadMarkdownYAML(path string) (*Config, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("read config %s: %w", path, err)
+	}
+	// Treat the whole file as the frontmatter block (no surrounding ---).
+	return parseFrontmatterBlock(string(data), path)
+}
+
+func parseFrontmatter(content, path string) (*Config, error) {
+	block, ok := extractFrontmatter(content)
+	if !ok {
+		// No frontmatter delimiters: treat whole file as a bare block.
+		return parseFrontmatterBlock(content, path)
+	}
+	return parseFrontmatterBlock(block, path)
+}
+
+// extractFrontmatter returns the YAML block between the first pair of
+// "---" delimiters and whether a frontmatter block was present.
+func extractFrontmatter(content string) (string, bool) {
+	trimmed := strings.TrimLeft(content, "\r\n\t ")
+	if !strings.HasPrefix(trimmed, "---") {
+		return "", false
+	}
+	// Skip the opening delimiter line.
+	rest := trimmed[3:]
+	rest = strings.TrimLeft(rest, "\r\n")
+	// Find the closing delimiter line.
+	idx := strings.Index(rest, "\n---")
+	if idx < 0 {
+		return "", false
+	}
+	return rest[:idx], true
+}
+
+// parseFrontmatterBlock parses a minimal YAML-ish block into *Config.
+// Supported shapes:
+//
+//	key: value
+//	node_capacity:
+//	  cpu: 4
+//	  memory_mb: 8192
+//
+// Comments (# ...) and blank lines are ignored. Quoted scalar values
+// ("..." or '...') are unwrapped. No flow collections, anchors, or
+// multi-line strings are supported — by design, to avoid adding a YAML
+// dependency for this small config surface.
+func parseFrontmatterBlock(block, path string) (*Config, error) {
+	cfg := &Config{}
+	var inCapacity bool
+
+	lines := strings.Split(block, "\n")
+	for lineNo, raw := range lines {
+		line := stripComment(raw)
+		if strings.TrimSpace(line) == "" {
+			continue
+		}
+
+		indent := countIndent(line)
+		trimmed := strings.TrimSpace(line)
+
+		// A top-level key (no leading indent).
+		if indent == 0 {
+			inCapacity = false
+			key, val, ok := splitKV(trimmed)
+			if !ok {
+				continue
+			}
+			if val == "" {
+				// key with no value → nested mapping header (e.g. node_capacity:)
+				if key == "node_capacity" {
+					cfg.NodeCapacity = &CapacityConfig{}
+					inCapacity = true
+				}
+				continue
+			}
+			applyScalar(cfg, key, val, path, lineNo)
+			continue
+		}
+
+		// Indented line under a nested mapping.
+		if inCapacity && cfg.NodeCapacity != nil {
+			key, val, hasVal := splitKV(trimmed)
+			if !hasVal {
+				continue
+			}
+			switch key {
+			case "cpu":
+				if n, err := strconv.Atoi(strings.TrimSpace(val)); err == nil {
+					cfg.NodeCapacity.CPU = n
+				}
+			case "memory_mb":
+				if n, err := strconv.Atoi(strings.TrimSpace(val)); err == nil {
+					cfg.NodeCapacity.MemoryMB = n
+				}
+			}
+		}
+	}
+	return cfg, nil
+}
+
+func applyScalar(cfg *Config, key, val, path string, lineNo int) {
+	val = strings.TrimSpace(val)
+	switch key {
+	case "db_path":
+		cfg.DBPath = unquote(val)
+	case "listen_addr":
+		cfg.ListenAddr = unquote(val)
+	case "ca_path":
+		cfg.CAPath = unquote(val)
+	case "server_cert_path":
+		cfg.ServerCertPath = unquote(val)
+	case "server_key_path":
+		cfg.ServerKeyPath = unquote(val)
+	}
+	_ = path
+	_ = lineNo
+}
+
+func splitKV(s string) (key, val string, ok bool) {
+	idx := strings.Index(s, ":")
+	if idx < 0 {
+		return "", "", false
+	}
+	key = strings.TrimSpace(s[:idx])
+	val = strings.TrimSpace(s[idx+1:])
+	if key == "" {
+		return "", "", false
+	}
+	return key, val, true
+}
+
+func countIndent(s string) int {
+	n := 0
+	for _, r := range s {
+		if r == ' ' || r == '\t' {
+			n++
+			continue
+		}
+		break
+	}
+	return n
+}
+
+func stripComment(s string) string {
+	// Strip inline comments not inside quotes. Minimal: only strip
+	// when the '#' is preceded by whitespace or at line start.
+	inSingle := false
+	inDouble := false
+	for i := 0; i < len(s); i++ {
+		c := s[i]
+		switch c {
+		case '\'':
+			if !inDouble {
+				inSingle = !inSingle
+			}
+		case '"':
+			if !inSingle {
+				inDouble = !inDouble
+			}
+		case '#':
+			if !inSingle && !inDouble {
+				if i == 0 || s[i-1] == ' ' || s[i-1] == '\t' {
+					return s[:i]
+				}
+			}
+		}
+	}
+	return s
+}
+
+func unquote(s string) string {
+	if len(s) >= 2 {
+		if (s[0] == '"' && s[len(s)-1] == '"') || (s[0] == '\'' && s[len(s)-1] == '\'') {
+			return s[1 : len(s)-1]
+		}
+	}
+	return s
+}
diff --git a/internal/config/markdown_test.go b/internal/config/markdown_test.go
new file mode 100644
index 0000000..c3f20ff
--- /dev/null
+++ b/internal/config/markdown_test.go
@@ -0,0 +1,186 @@
+package config
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func writeTestFile(t *testing.T, dir, name, content string) string {
+	t.Helper()
+	p := filepath.Join(dir, name)
+	if err := os.WriteFile(p, []byte(content), 0644); err != nil {
+		t.Fatalf("write %s: %v", p, err)
+	}
+	return p
+}
+
+const exampleMarkdown = `---
+listen_addr: "127.0.0.1:9999"
+db_path: "/tmp/orca/test.db"
+ca_path: "/tmp/orca/ca.crt"
+server_cert_path: "/tmp/orca/server.crt"
+server_key_path: "/tmp/orca/server.key"
+node_capacity:
+  cpu: 4
+  memory_mb: 8192
+---
+
+# Orca config
+
+This is prose body and is ignored by the loader.
+`
+
+func TestLoadMarkdown_Full(t *testing.T) {
+	p := writeTestFile(t, t.TempDir(), "config.md", exampleMarkdown)
+	cfg, err := LoadMarkdown(p)
+	if err != nil {
+		t.Fatalf("LoadMarkdown: %v", err)
+	}
+	if cfg.DBPath != "/tmp/orca/test.db" {
+		t.Errorf("DBPath=%q", cfg.DBPath)
+	}
+	if cfg.ListenAddr != "127.0.0.1:9999" {
+		t.Errorf("ListenAddr=%q", cfg.ListenAddr)
+	}
+	if cfg.CAPath != "/tmp/orca/ca.crt" {
+		t.Errorf("CAPath=%q", cfg.CAPath)
+	}
+	if cfg.ServerCertPath != "/tmp/orca/server.crt" {
+		t.Errorf("ServerCertPath=%q", cfg.ServerCertPath)
+	}
+	if cfg.ServerKeyPath != "/tmp/orca/server.key" {
+		t.Errorf("ServerKeyPath=%q", cfg.ServerKeyPath)
+	}
+	if cfg.NodeCapacity == nil {
+		t.Fatal("NodeCapacity nil")
+	}
+	if cfg.NodeCapacity.CPU != 4 {
+		t.Errorf("CPU=%d", cfg.NodeCapacity.CPU)
+	}
+	if cfg.NodeCapacity.MemoryMB != 8192 {
+		t.Errorf("MemoryMB=%d", cfg.NodeCapacity.MemoryMB)
+	}
+}
+
+func TestLoadMarkdown_NoFrontmatter(t *testing.T) {
+	// No delimiters: whole file treated as a bare YAML block.
+	body := "listen_addr: 0.0.0.0:1234\ndb_path: /x/y.db\n"
+	p := writeTestFile(t, t.TempDir(), "config.md", body)
+	cfg, err := LoadMarkdown(p)
+	if err != nil {
+		t.Fatalf("LoadMarkdown: %v", err)
+	}
+	if cfg.ListenAddr != "0.0.0.0:1234" {
+		t.Errorf("ListenAddr=%q", cfg.ListenAddr)
+	}
+	if cfg.DBPath != "/x/y.db" {
+		t.Errorf("DBPath=%q", cfg.DBPath)
+	}
+}
+
+func TestLoadMarkdown_OnlyBody(t *testing.T) {
+	body := `---
+---
+
+# Just prose, no keys
+`
+	p := writeTestFile(t, t.TempDir(), "config.md", body)
+	cfg, err := LoadMarkdown(p)
+	if err != nil {
+		t.Fatalf("LoadMarkdown: %v", err)
+	}
+	if cfg.DBPath != "" || cfg.ListenAddr != "" || cfg.NodeCapacity != nil {
+		t.Errorf("expected zero config, got %+v", cfg)
+	}
+}
+
+func TestLoadMarkdown_CommentsAndBlanks(t *testing.T) {
+	body := `---
+# a comment
+listen_addr: "127.0.0.1:9999"
+
+db_path: "/tmp/orca/test.db"  # inline comment
+
+node_capacity:
+  cpu: 4   # cores
+  memory_mb: 8192
+---
+`
+	p := writeTestFile(t, t.TempDir(), "config.md", body)
+	cfg, err := LoadMarkdown(p)
+	if err != nil {
+		t.Fatalf("LoadMarkdown: %v", err)
+	}
+	if cfg.ListenAddr != "127.0.0.1:9999" {
+		t.Errorf("ListenAddr=%q", cfg.ListenAddr)
+	}
+	if cfg.DBPath != "/tmp/orca/test.db" {
+		t.Errorf("DBPath=%q", cfg.DBPath)
+	}
+	if cfg.NodeCapacity == nil || cfg.NodeCapacity.CPU != 4 || cfg.NodeCapacity.MemoryMB != 8192 {
+		t.Errorf("NodeCapacity=%+v", cfg.NodeCapacity)
+	}
+}
+
+func TestLoadMarkdownYAML_Bare(t *testing.T) {
+	body := "listen_addr: 0.0.0.0:5555\ndb_path: /bare.db\nnode_capacity:\n  cpu: 2\n  memory_mb: 4096\n"
+	p := writeTestFile(t, t.TempDir(), "config.yaml", body)
+	cfg, err := LoadMarkdownYAML(p)
+	if err != nil {
+		t.Fatalf("LoadMarkdownYAML: %v", err)
+	}
+	if cfg.ListenAddr != "0.0.0.0:5555" {
+		t.Errorf("ListenAddr=%q", cfg.ListenAddr)
+	}
+	if cfg.DBPath != "/bare.db" {
+		t.Errorf("DBPath=%q", cfg.DBPath)
+	}
+	if cfg.NodeCapacity == nil || cfg.NodeCapacity.CPU != 2 || cfg.NodeCapacity.MemoryMB != 4096 {
+		t.Errorf("NodeCapacity=%+v", cfg.NodeCapacity)
+	}
+}
+
+func TestLoadMarkdown_ReadError(t *testing.T) {
+	missing := filepath.Join(t.TempDir(), "nope.md")
+	if _, err := LoadMarkdown(missing); err == nil {
+		t.Fatal("expected error for missing file")
+	}
+}
+
+func TestExtractFrontmatter(t *testing.T) {
+	cases := []struct {
+		name    string
+		input   string
+		block   string
+		present bool
+	}{
+		{"standard", "---\nkey: val\n---\nbody", "key: val", true},
+		{"leading-blanks", "\n\n---\nkey: val\n---\n", "key: val", true},
+		{"no-delimiters", "key: val\n", "key: val", false},
+		{"only-open", "---\nkey: val\n", "key: val", false},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			block, ok := extractFrontmatter(tc.input)
+			if ok != tc.present {
+				t.Errorf("present=%v want %v", ok, tc.present)
+			}
+			if tc.present && block != tc.block {
+				t.Errorf("block=%q want %q", block, tc.block)
+			}
+		})
+	}
+}
+
+func TestUnquote(t *testing.T) {
+	if got, want := unquote(`"hello"`), "hello"; got != want {
+		t.Errorf("unquote double = %q want %q", got, want)
+	}
+	if got, want := unquote(`'hello'`), "hello"; got != want {
+		t.Errorf("unquote single = %q want %q", got, want)
+	}
+	if got, want := unquote("bare"), "bare"; got != want {
+		t.Errorf("unquote bare = %q want %q", got, want)
+	}
+}
diff --git a/internal/paths/paths.go b/internal/paths/paths.go
new file mode 100644
index 0000000..eea7041
--- /dev/null
+++ b/internal/paths/paths.go
@@ -0,0 +1,121 @@
+// Package paths resolves on-disk locations for the v0.9 multi-namespace
+// filesystem layout (R-002). It is the canonical source of truth for
+// cluster-wide, per-namespace, and CLI-cache paths.
+//
+// The v0.8 internal/certpaths package is preserved as a thin shim that
+// returns the legacy flat-layout paths during the v0.9 dual-write window
+// (REQ-090). New code should use internal/paths, NOT certpaths.
+package paths
+
+import (
+	"os"
+	"path/filepath"
+)
+
+const (
+	defaultHomeSubdir = ".orca"
+	clusterDirName    = "cluster"
+	defaultNamespace  = "_defaults"
+)
+
+// Root returns the ORCA home directory. It honors $ORCA_HOME for
+// testability; otherwise it defaults to ~/.orca. An empty $ORCA_HOME is
+// treated as unset.
+func Root() string {
+	if p := os.Getenv("ORCA_HOME"); p != "" {
+		return p
+	}
+	home, _ := os.UserHomeDir()
+	return filepath.Join(home, defaultHomeSubdir)
+}
+
+// ClusterDir returns the cluster-wide directory: Root()/cluster.
+// Cluster-wide artifacts (CA, master key, peers, txns, known_hosts, SSH
+// keys) live here and are NOT scoped to a workload namespace (R-002).
+func ClusterDir() string { return filepath.Join(Root(), clusterDirName) }
+
+// NamespaceDir returns the directory for a namespace: Root()/<ns>.
+// Use DefaultNamespace() for the implicit root namespace (R-002 D-159).
+func NamespaceDir(ns string) string { return filepath.Join(Root(), ns) }
+
+// NSDb returns the SQLite database path for a namespace:
+// NamespaceDir(ns)/db/orca.db.
+func NSDb(ns string) string { return filepath.Join(NamespaceDir(ns), "db", "orca.db") }
+
+// NSEnv returns the .env path for a namespace: NamespaceDir(ns)/.env.
+func NSEnv(ns string) string { return filepath.Join(NamespaceDir(ns), ".env") }
+
+// NSSecrets returns the encrypted secrets env path for a namespace:
+// NamespaceDir(ns)/.env.secrets.
+func NSSecrets(ns string) string { return filepath.Join(NamespaceDir(ns), ".env.secrets") }
+
+// NSJobs returns the jobs directory for a namespace: NamespaceDir(ns)/jobs.
+func NSJobs(ns string) string { return filepath.Join(NamespaceDir(ns), "jobs") }
+
+// NSAlloc returns the allocation directory for a namespace:
+// NamespaceDir(ns)/alloc.
+func NSAlloc(ns string) string { return filepath.Join(NamespaceDir(ns), "alloc") }
+
+// NSMd returns the namespace Markdown doc path (R-014):
+// NamespaceDir(ns)/ns.md.
+func NSMd(ns string) string { return filepath.Join(NamespaceDir(ns), "ns.md") }
+
+// DefaultNamespace returns the implicit root namespace name (R-002 D-159).
+func DefaultNamespace() string { return defaultNamespace }
+
+// CACertPath returns the v0.9 cluster CA cert path:
+// ClusterDir()/ca.crt (D-101). The v0.8 internal CA still writes to
+// Root()/ca.crt; the move happens in v0.10-P14.
+func CACertPath() string { return filepath.Join(ClusterDir(), "ca.crt") }
+
+// CAKeyPath returns the v0.9 cluster CA key path:
+// ClusterDir()/ca.key.
+func CAKeyPath() string { return filepath.Join(ClusterDir(), "ca.key") }
+
+// MasterKeyPath returns the AES-256-GCM root master key path
+// (R-011, mode 0600): ClusterDir()/master.key. Not generated until
+// v0.10-P03.
+func MasterKeyPath() string { return filepath.Join(ClusterDir(), "master.key") }
+
+// CacheDB returns the CLI-side cache database path (R-008):
+// Root()/orca_cache.db. Not created until v0.9-P0a2.
+func CacheDB() string { return filepath.Join(Root(), "orca_cache.db") }
+
+// TxnDir returns the cluster transaction log directory (R-016):
+// ClusterDir()/txns.
+func TxnDir() string { return filepath.Join(ClusterDir(), "txns") }
+
+// PeersDir returns the cluster peers directory: ClusterDir()/peers.
+func PeersDir() string { return filepath.Join(ClusterDir(), "peers") }
+
+// PeerDir returns the directory for a single peer host:
+// PeersDir()/host.
+func PeerDir(host string) string { return filepath.Join(PeersDir(), host) }
+
+// KnownHostsPath returns the SSH known_hosts path (D-035):
+// ClusterDir()/known_hosts.
+func KnownHostsPath() string { return filepath.Join(ClusterDir(), "known_hosts") }
+
+// SSHKeyPath returns the orca SSH private key path:
+// ClusterDir()/orca_ssh_key (D-037).
+func SSHKeyPath() string { return filepath.Join(ClusterDir(), "orca_ssh_key") }
+
+// SSHPubPath returns the orca SSH public key path:
+// ClusterDir()/orca_ssh_key.pub.
+func SSHPubPath() string { return filepath.Join(ClusterDir(), "orca_ssh_key.pub") }
+
+// ServerCertPath returns the legacy server cert path (legacy compat):
+// ClusterDir()/server.crt. step-ca will replace this in a later phase.
+func ServerCertPath() string { return filepath.Join(ClusterDir(), "server.crt") }
+
+// ServerKeyPath returns the legacy server key path (legacy compat):
+// ClusterDir()/server.key. step-ca will replace this in a later phase.
+func ServerKeyPath() string { return filepath.Join(ClusterDir(), "server.key") }
+
+// ConfigPath returns the new Markdown-frontmatter config path (R-014):
+// ClusterDir()/config.md. The legacy HCL path is ClusterDir()/config.hcl.
+func ConfigPath() string { return filepath.Join(ClusterDir(), "config.md") }
+
+// LegacyHCLConfigPath returns the legacy HCL config path:
+// ClusterDir()/config.hcl.
+func LegacyHCLConfigPath() string { return filepath.Join(ClusterDir(), "config.hcl") }
diff --git a/internal/paths/paths_test.go b/internal/paths/paths_test.go
new file mode 100644
index 0000000..6b4067e
--- /dev/null
+++ b/internal/paths/paths_test.go
@@ -0,0 +1,209 @@
+package paths
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestRoot_HonorsORCAHOME(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("ORCA_HOME", dir)
+	if got, want := Root(), dir; got != want {
+		t.Errorf("Root() = %q, want %q", got, want)
+	}
+}
+
+func TestRoot_EmptyORCAHOMEFallsBack(t *testing.T) {
+	t.Setenv("ORCA_HOME", "")
+	home, err := os.UserHomeDir()
+	if err != nil {
+		t.Skipf("os.UserHomeDir: %v", err)
+	}
+	want := filepath.Join(home, defaultHomeSubdir)
+	if got := Root(); got != want {
+		t.Errorf("Root() with empty ORCA_HOME = %q, want %q", got, want)
+	}
+}
+
+func TestRoot_UnsetORCAHOMEFallsBack(t *testing.T) {
+	os.Unsetenv("ORCA_HOME")
+	home, err := os.UserHomeDir()
+	if err != nil {
+		t.Skipf("os.UserHomeDir: %v", err)
+	}
+	want := filepath.Join(home, defaultHomeSubdir)
+	got := Root()
+	if got != want {
+		t.Errorf("Root() default = %q, want %q", got, want)
+	}
+	if !strings.HasPrefix(got, home) {
+		t.Errorf("Root() default %q does not start with home %q", got, home)
+	}
+}
+
+func TestRoot_RelativeORCAHOME(t *testing.T) {
+	t.Setenv("ORCA_HOME", "relative/orca/home")
+	if got, want := Root(), "relative/orca/home"; got != want {
+		t.Errorf("Root() relative = %q, want %q", got, want)
+	}
+}
+
+func TestDefaultNamespace(t *testing.T) {
+	if got, want := DefaultNamespace(), "_defaults"; got != want {
+		t.Errorf("DefaultNamespace() = %q, want %q", got, want)
+	}
+}
+
+func TestClusterDir(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("ORCA_HOME", dir)
+	got := ClusterDir()
+	want := filepath.Join(dir, "cluster")
+	if got != want {
+		t.Errorf("ClusterDir() = %q, want %q", got, want)
+	}
+	if !strings.HasPrefix(got, Root()+string(filepath.Separator)) {
+		t.Errorf("ClusterDir() %q not under Root() %q", got, Root())
+	}
+}
+
+func TestNamespaceDir(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("ORCA_HOME", dir)
+	ns := "prod"
+	got := NamespaceDir(ns)
+	want := filepath.Join(dir, ns)
+	if got != want {
+		t.Errorf("NamespaceDir(%q) = %q, want %q", ns, got, want)
+	}
+	if !strings.HasPrefix(got, Root()+string(filepath.Separator)) {
+		t.Errorf("NamespaceDir() %q not under Root() %q", got, Root())
+	}
+}
+
+func TestNamespacePaths(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("ORCA_HOME", dir)
+	ns := "prod"
+	nsDir := NamespaceDir(ns)
+
+	cases := []struct {
+		name string
+		got  string
+		want string
+	}{
+		{"NSDb", NSDb(ns), filepath.Join(nsDir, "db", "orca.db")},
+		{"NSEnv", NSEnv(ns), filepath.Join(nsDir, ".env")},
+		{"NSSecrets", NSSecrets(ns), filepath.Join(nsDir, ".env.secrets")},
+		{"NSJobs", NSJobs(ns), filepath.Join(nsDir, "jobs")},
+		{"NSAlloc", NSAlloc(ns), filepath.Join(nsDir, "alloc")},
+		{"NSMd", NSMd(ns), filepath.Join(nsDir, "ns.md")},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			if tc.got != tc.want {
+				t.Errorf("%s(%q) = %q, want %q", tc.name, ns, tc.got, tc.want)
+			}
+			if !strings.HasPrefix(tc.got, nsDir+string(filepath.Separator)) {
+				t.Errorf("%s() %q not under NamespaceDir() %q", tc.name, tc.got, nsDir)
+			}
+		})
+	}
+}
+
+func TestDefaultNamespacePaths(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("ORCA_HOME", dir)
+	ns := DefaultNamespace()
+	nsDir := NamespaceDir(ns)
+
+	if got, want := NSDb(ns), filepath.Join(nsDir, "db", "orca.db"); got != want {
+		t.Errorf("NSDb(_defaults) = %q, want %q", got, want)
+	}
+	if got, want := NSEnv(ns), filepath.Join(nsDir, ".env"); got != want {
+		t.Errorf("NSEnv(_defaults) = %q, want %q", got, want)
+	}
+}
+
+func TestClusterPaths(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("ORCA_HOME", dir)
+	cDir := ClusterDir()
+
+	cases := []struct {
+		name string
+		got  string
+		want string
+	}{
+		{"CACertPath", CACertPath(), filepath.Join(cDir, "ca.crt")},
+		{"CAKeyPath", CAKeyPath(), filepath.Join(cDir, "ca.key")},
+		{"MasterKeyPath", MasterKeyPath(), filepath.Join(cDir, "master.key")},
+		{"KnownHostsPath", KnownHostsPath(), filepath.Join(cDir, "known_hosts")},
+		{"SSHKeyPath", SSHKeyPath(), filepath.Join(cDir, "orca_ssh_key")},
+		{"SSHPubPath", SSHPubPath(), filepath.Join(cDir, "orca_ssh_key.pub")},
+		{"ServerCertPath", ServerCertPath(), filepath.Join(cDir, "server.crt")},
+		{"ServerKeyPath", ServerKeyPath(), filepath.Join(cDir, "server.key")},
+		{"ConfigPath", ConfigPath(), filepath.Join(cDir, "config.md")},
+		{"LegacyHCLConfigPath", LegacyHCLConfigPath(), filepath.Join(cDir, "config.hcl")},
+		{"TxnDir", TxnDir(), filepath.Join(cDir, "txns")},
+		{"PeersDir", PeersDir(), filepath.Join(cDir, "peers")},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			if tc.got != tc.want {
+				t.Errorf("%s() = %q, want %q", tc.name, tc.got, tc.want)
+			}
+			if !strings.HasPrefix(tc.got, cDir+string(filepath.Separator)) {
+				t.Errorf("%s() %q not under ClusterDir() %q", tc.name, tc.got, cDir)
+			}
+		})
+	}
+}
+
+func TestPeerDir(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("ORCA_HOME", dir)
+	host := "node1.example.com"
+	got := PeerDir(host)
+	want := filepath.Join(PeersDir(), host)
+	if got != want {
+		t.Errorf("PeerDir(%q) = %q, want %q", host, got, want)
+	}
+	if !strings.HasPrefix(got, PeersDir()+string(filepath.Separator)) {
+		t.Errorf("PeerDir() %q not under PeersDir() %q", got, PeersDir())
+	}
+}
+
+func TestCacheDB(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("ORCA_HOME", dir)
+	got := CacheDB()
+	want := filepath.Join(dir, "orca_cache.db")
+	if got != want {
+		t.Errorf("CacheDB() = %q, want %q", got, want)
+	}
+	if !strings.HasPrefix(got, Root()+string(filepath.Separator)) {
+		t.Errorf("CacheDB() %q not under Root() %q", got, Root())
+	}
+}
+
+func TestPathSeparatorsOSAppropriate(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("ORCA_HOME", dir)
+	// Every returned path must use the OS separator (filepath.Join).
+	sep := string(filepath.Separator)
+	for _, p := range []string{
+		ClusterDir(), NamespaceDir("ns"), NSDb("ns"), NSEnv("ns"),
+		NSSecrets("ns"), NSJobs("ns"), NSAlloc("ns"), NSMd("ns"),
+		CACertPath(), CAKeyPath(), MasterKeyPath(), CacheDB(),
+		TxnDir(), PeersDir(), PeerDir("h"), KnownHostsPath(),
+		SSHKeyPath(), SSHPubPath(), ServerCertPath(), ServerKeyPath(),
+		ConfigPath(), LegacyHCLConfigPath(),
+	} {
+		if !strings.Contains(p, sep) {
+			t.Errorf("path %q lacks OS separator %q (not joined?)", p, sep)
+		}
+	}
+}
diff --git a/internal/proxmox/bootstrap.go b/internal/proxmox/bootstrap.go
index 25ecfdd..1bd67cc 100644
--- a/internal/proxmox/bootstrap.go
+++ b/internal/proxmox/bootstrap.go
@@ -289,6 +289,11 @@ func TOFUHostKeyCallback(addr string, capturedKey *ssh.PublicKey) (ssh.HostKeyCa
 		if errors.As(err, &keyErr) && len(keyErr.Want) == 0 {
 			line := knownhosts.Line([]string{knownhosts.Normalize(addr)}, key)
 			path := certpaths.KnownHostsPath()
+			release, lockErr := security.Flock(path)
+			if lockErr != nil {
+				return fmt.Errorf("tofu lock known_hosts: %w", lockErr)
+			}
+			defer release()
 			existing, readErr := os.ReadFile(path)
 			if readErr != nil && !os.IsNotExist(readErr) {
 				return fmt.Errorf("tofu read known_hosts: %w", readErr)
@@ -481,6 +486,11 @@ func ResetHostKey(host string) error {
 		return fmt.Errorf("ResetHostKey: host is required")
 	}
 	path := certpaths.KnownHostsPath()
+	release, lockErr := security.Flock(path)
+	if lockErr != nil {
+		return fmt.Errorf("ResetHostKey: lock known_hosts: %w", lockErr)
+	}
+	defer release()
 	existing, err := os.ReadFile(path)
 	if err != nil {
 		if os.IsNotExist(err) {
diff --git a/internal/security/flock.go b/internal/security/flock.go
new file mode 100644
index 0000000..df33258
--- /dev/null
+++ b/internal/security/flock.go
@@ -0,0 +1,27 @@
+package security
+
+import (
+	"os"
+	"syscall"
+)
+
+// Flock acquires an exclusive advisory lock on the file at path, creating it
+// if missing. Returns a release function that MUST be called (deferred) to
+// release the lock and close the file descriptor. Used by the known_hosts
+// read-modify-write paths (TOFUHostKeyCallback capture + ResetHostKey) to
+// prevent concurrent writers under v0.9's parallel SSH fan-out (REQ-063,
+// deferred P1 from REVIEW_v0.8 A2).
+func Flock(path string) (release func(), err error) {
+	f, err := os.OpenFile(path, os.O_CREATE|os.O_RDWR, 0o600)
+	if err != nil {
+		return nil, err
+	}
+	if err := syscall.Flock(int(f.Fd()), syscall.LOCK_EX); err != nil {
+		f.Close()
+		return nil, err
+	}
+	return func() {
+		_ = syscall.Flock(int(f.Fd()), syscall.LOCK_UN)
+		_ = f.Close()
+	}, nil
+}
diff --git a/internal/security/flock_test.go b/internal/security/flock_test.go
new file mode 100644
index 0000000..f44bcc5
--- /dev/null
+++ b/internal/security/flock_test.go
@@ -0,0 +1,61 @@
+package security
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestFlock_acquireAndRelease(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "test.lock")
+
+	release, err := Flock(path)
+	if err != nil {
+		t.Fatalf("Flock: %v", err)
+	}
+	if _, statErr := os.Stat(path); statErr != nil {
+		t.Fatalf("lock file not created: %v", statErr)
+	}
+	release()
+}
+
+func TestFlock_reentrantAfterRelease(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "test.lock")
+
+	r1, err := Flock(path)
+	if err != nil {
+		t.Fatalf("first Flock: %v", err)
+	}
+	r1()
+
+	r2, err := Flock(path)
+	if err != nil {
+		t.Fatalf("second Flock after release: %v", err)
+	}
+	r2()
+}
+
+func TestFlock_concurrentBlocks(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "test.lock")
+
+	r1, err := Flock(path)
+	if err != nil {
+		t.Fatalf("first Flock: %v", err)
+	}
+	defer r1()
+
+	done := make(chan error, 1)
+	go func() {
+		_, err := Flock(path)
+		done <- err
+	}()
+
+	select {
+	case <-done:
+		t.Fatal("second Flock should block while first holds the lock")
+	default:
+	}
+}