feat(P12): --type linux SSH-join + UAT plan + signoff script (REQ-161..163)

--type linux (REQ-161):
- internal/linux/bootstrap.go: SSH bootstrap for generic Linux workers
  (orcas pubkey, system user, drift-events dir; no PVE role/sudoers)
- internal/cli/node.go: joinLinux function + --type linux dispatch
- peer-setup kept as documented fallback

UAT plan (REQ-162):
- docs/uat.md: 3-host topology (lead Ubuntu + pve01 Proxmox + worker01
  Ubuntu), 22 step-by-step commands, 35-claim matrix, Proxmox
  prerequisite + alternative 3xUbuntu path (C-48), signoff procedure

UAT signoff script (REQ-163, C-47):
- scripts/uat-signoff.sh: 35 idempotent read-only assertions, exit 0
  iff all pass. Includes 4 critical-path assertions: job deploys to
  remote, ACL deny-by-default, seal/unseal round-trip, OIDC health
- scripts/uat-smoke.sh: 13 CI-tested pure-CLI assertions for .coreci.yml

Tests: node join --type linux test, fingerprint test updated, smoke
test all 13 pass.

---ci---
project: orca
phase: 12
milestone: v0.13
status: complete
requirements:
  covered: [161, 162, 163]
---/ci---

internal/cli/node.go

@@ -15,6 +15,7 @@ import (
 	"github.com/spf13/cobra"
 
 	"git.cloudinit.dev/coreci/orca/internal/certpaths"
+	"git.cloudinit.dev/coreci/orca/internal/linux"
 	"git.cloudinit.dev/coreci/orca/internal/engine"
 	"git.cloudinit.dev/coreci/orca/internal/model"
 	"git.cloudinit.dev/coreci/orca/internal/proxmox"
@@ -73,16 +74,22 @@ var nodeJoinCmd = &cobra.Command{
 
 Node types (via --type):
   localhost (default): register a local or Linux node (existing behavior)
+  linux:               SSH-bootstrap a remote generic Linux worker
+                       (Ubuntu/Debian/Alpine; deploys orca pubkey, creates orca
+                       user + drift-events dir; requires --host + --ssh-key)
   proxmox:             SSH-bootstrap a remote Proxmox VE 8/9 host
                        (deploys orca pubkey, creates orca user + PVE role +
                        sudoers allowlist; requires --host + --ssh-key (R-021: no passwords))`,
 	RunE: func(cmd *cobra.Command, args []string) error {
-		if joinHostKeyFP != "" && joinType != "proxmox" {
-			return fmt.Errorf("--host-key-fingerprint requires --type proxmox today")
+		if joinHostKeyFP != "" && joinType != "proxmox" && joinType != "linux" {
+			return fmt.Errorf("--host-key-fingerprint requires --type proxmox or --type linux")
 		}
 		if joinType == "proxmox" {
 			return joinProxmox(cmd)
 		}
+		if joinType == "linux" {
+			return joinLinux(cmd)
+		}
 		return joinLocal(cmd)
 	},
 }
@@ -217,6 +224,70 @@ func joinProxmox(cmd *cobra.Command) error {
 	return nil
 }
 
+// joinLinux bootstraps a remote generic Linux worker via SSH and
+// registers it as an orca node (REQ-161, P12). Uses SSH key auth
+// (R-021: no passwords).
+func joinLinux(cmd *cobra.Command) error {
+	if joinHost == "" {
+		return fmt.Errorf("--host is required for --type linux")
+	}
+	sshKeyPath := joinSSHKey
+	if sshKeyPath == "" {
+		sshKeyPath = certpaths.SSHKeyPath()
+	}
+	if sshKeyPath == "" {
+		return fmt.Errorf("SSH key path is required for --type linux (R-021: no passwords; use --ssh-key or pre-stage the orca key)")
+	}
+
+	ctx, cancel := context.WithTimeout(cmd.Context(), 60*time.Second)
+	defer cancel()
+
+	result, err := linux.BootstrapLinux(ctx, linux.Options{
+		Host:               joinHost,
+		SSHUser:            joinSSHUser,
+		SSHKeyPath:         sshKeyPath,
+		OrcaUser:           proxmoxUser,
+		SSHPort:            joinSSHPort,
+		HostKeyFingerprint: joinHostKeyFP,
+		Logger:             newLogger(),
+	})
+	if err != nil {
+		return fmt.Errorf("linux bootstrap: %w", err)
+	}
+
+	registry, closer, err := nodeRegistry()
+	if err != nil {
+		return err
+	}
+	defer closer()
+
+	regCtx, regCancel := context.WithTimeout(ctx, 5*time.Second)
+	defer regCancel()
+
+	node := &model.Node{
+		ID:       uuid.NewString(),
+		Name:     result.NodeName,
+		Address:  result.NodeAddress,
+		State:    model.NodeStateReady,
+		JoinedAt: time.Now().UTC(),
+		LastSeen: time.Now().UTC(),
+		Kind:     string(model.NodeKindLinux),
+		OS:       "linux",
+	}
+	if err := registry.Join(regCtx, node); err != nil {
+		return fmt.Errorf("register linux node: %w", err)
+	}
+	cacheInvalidate(cacheNodeClass)
+	if jsonOutput {
+		return printJSON(node)
+	}
+	fmt.Fprintf(cmd.OutOrStdout(), "\xe2\x9c\x93 Linux worker joined: %s (%s) at %s\n", node.ID, node.Name, node.Address)
+	if result.HostKeyFingerprint != "" {
+		fmt.Fprintf(cmd.OutOrStdout(), "  host key: %s\n", result.HostKeyFingerprint)
+	}
+	return nil
+}
+
 var nodeLeaveCmd = &cobra.Command{
 	Use:   "leave [node-id]",
 	Short: "Remove a node from the orca registry",