feat(P01): unified namespace root via ORCA_HOME + --system flag

REQ-041: ORCA_HOME is now the single namespace root for all components
  (db, certs, init, daemon). store.Open("") and init command both
  route through certpaths.Dir()/DBPath() instead of hardcoding ~/.orca.
  Backward compatible: empty ORCA_HOME -> ~/.orca.

REQ-042: --system persistent flag on rootCmd sets ORCA_HOME=/root/.orca
  via PersistentPreRunE. Errors on conflict with pre-set ORCA_HOME.

Tests: 7 new tests in namespace_test.go (default, ORCA_HOME override,
  --system sets root, conflict detection, init --json, flag registered).
  Full suite passes (no regressions).

Docs: docs/namespace.md covers default, ORCA_HOME, --system, ORCA_DB,
  resolution order, and path layout tables.

---ci---
project: orca
phase: 1
milestone: v0.5
status: verify
---/ci---

internal/cli/init.go

@@ -3,21 +3,18 @@ package cli
 import (
 	"fmt"
 	"os"
-	"path/filepath"
 
 	"github.com/spf13/cobra"
+
+	"git.cloudinit.dev/coreci/orca/internal/certpaths"
 )
 
 var initCmd = &cobra.Command{
 	Use:   "init",
 	Short: "Initialize local orca state directory",
-	Long:  "Create the local orca state directory at ~/.orca/ and write a default config file.",
+	Long:  "Create the local orca state directory (honors $ORCA_HOME; defaults to ~/.orca) and write a default config file.",
 	RunE: func(cmd *cobra.Command, args []string) error {
-		home, err := os.UserHomeDir()
-		if err != nil {
-			return fmt.Errorf("get home dir: %w", err)
-		}
-		orcaDir := filepath.Join(home, ".orca")
+		orcaDir := certpaths.Dir()
 		if err := os.MkdirAll(orcaDir, 0o755); err != nil {
 			return fmt.Errorf("create orca dir: %w", err)
 		}