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---

examples/full-stack/README.md

@@ -62,7 +62,7 @@ a localhost node.
 orca node join --type proxmox --host 192.168.1.100 --ssh-user root
 
 # Join a second node
-ORCA_PROXMOX_PASSWORD=secret orca node join --type proxmox --host 192.168.1.101
+orca node join --type proxmox --host 192.168.1.101 --ssh-key ~/.ssh/orca_ed25519
 ```
 
 ### Step 3: Declare node capacity