feat(P02): Proxmox SSH join + OrcaOperator role + sudoers

orca node join --type proxmox bootstraps a remote Proxmox VE 8/9 host
via SSH (REQ-050, REQ-051). The password is used only for initial auth;
subsequent access uses the deployed orca SSH key (D-031).

Changes:
- go.mod: add golang.org/x/crypto v0.54.0 (ssh + ssh/knownhosts + ed25519)
  bump x/sys to v0.47.0, add x/term (indirect)
- internal/certpaths: SSHKeyPath, SSHPubPath, KnownHostsPath (D-037)
- internal/security/sshkey.go: GenerateOrLoadSSHKey (Ed25519, PKCS8 PEM,
  0600/0644 modes, idempotent load per D-036)
- internal/proxmox/bootstrap.go: BootstrapProxmox SSH dance:
  1. Generate/load SSH key
  2. SSH dial (password + knownhosts.New TOFU per D-035)
  3. Deploy pubkey to ~orca/.ssh/authorized_keys (idempotent)
  4. useradd -m orca (idempotent)
  5. pveum role add OrcaOperator --privs 'VM.Audit Datastore.AllocateSpace SDN.Use'
  6. pveum user add orca@pam (AD-019: PAM realm, not @pve)
  7. pveum acl modify / -user orca@pam -role OrcaOperator
  8. Write /etc/sudoers.d/orca (AD-020: NOEXEC on pct/qm, no NOEXEC on
     apt-get/dpkg, pvesh EXCLUDED — API execute bypasses NOEXEC)
  9. visudo -cf validation (abort on failure)
  All steps idempotent; audit-logged.
- internal/cli/node.go: --type/--host/--ssh-user/--password/--ssh-port/
  --proxmox-user/--proxmox-role flags; joinProxmox() wires to
  proxmox.BootstrapProxmox + registers node with kind=proxmox, os=pve.
  Password zeroed after use (D-031).
- tests: sshkey generate/load round-trip, idempotency, file modes;
  proxmox sudoers content (NOEXEC/NOPASSWD/pvesh-excluded),
  privilege set, validation; node join flag wiring

---ci---
project: orca
phase: 2
milestone: v0.6
status: execute
---/ci---

internal/certpaths/certpaths.go

@@ -46,3 +46,19 @@ func DBPath() string {
 	}
 	return filepath.Join(Dir(), "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") }
+
+// 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") }
+
+// 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") }