fix(A): bootstrap plumbing — init creates SSH key + known_hosts + master key (REQ-164)

Fixes UAT issues 1, 8, 9, 12C, 13:
- orca init: generates SSH keypair (GenerateOrLoadSSHKey), creates
  empty known_hosts (0600), generates master key (GenerateMasterKey +
  SaveMasterKey). All were missing from runInit — every downstream
  SSH/secrets/cluster operation failed on a fresh init.
- TOFUHostKeyCallbackPath: creates known_hosts file if it doesn't exist
  (defense-in-depth alongside init)
- Linux bootstrap: replaces buggy inline TOFU with
  proxmox.TOFUHostKeyCallbackPath (first-connect key capture works)
- --type flag help: includes "linux" (was "localhost or proxmox")
- doctor network: SSH exec probe (was HTTP /healthz to :8443 — no
  daemon in SSH-push model R-001)

---ci---
project: orca
milestone: v0.12.18
phase: A
status: complete
requirements:
  covered: [164]
---/ci---

internal/cli/node.go

@@ -503,7 +503,7 @@ func init() {
 	nodeJoinCmd.Flags().StringVar(&joinName, "name", "", "node name (required for --type localhost)")
 	nodeJoinCmd.Flags().StringVar(&joinAddr, "addr", "", "node address (default localhost:8443)")
 	nodeJoinCmd.Flags().StringVar(&joinCAFinger, "ca-fingerprint", "", "pin CA cert SHA-256 (REQ-026); fails if on-disk CA doesn't match")
-	nodeJoinCmd.Flags().StringVar(&joinType, "type", "localhost", "node type: localhost (default) or proxmox (SSH bootstrap)")
+	nodeJoinCmd.Flags().StringVar(&joinType, "type", "localhost", "node type: localhost (default), proxmox, or linux (SSH bootstrap)")
 	nodeJoinCmd.Flags().StringVar(&joinHost, "host", "", "proxmox host address (IP/hostname, no port; required for --type proxmox)")
 	nodeJoinCmd.Flags().StringVar(&joinSSHUser, "ssh-user", "root", "SSH username for proxmox bootstrap (default root)")
 	nodeJoinCmd.Flags().StringVar(&joinSSHKey, "ssh-key", "", "SSH private key path for proxmox bootstrap (R-021: no passwords; default: orca key)")