fix(P07): remove all password/token paths (REQ-146, R-021, C-34) -- BREAKING

---ci---
project: orca
phase: 7
milestone: v0.12
status: execute
---/ci---

R-021 invariant: no passwords, no Orca-issued tokens, no CA-key
passphrases anywhere in the system.

Removed:
- proxmox/bootstrap.go: ssh.Password auth -> ssh.PublicKeys (key-based).
  --password/ removed from node join; replaced
  with --ssh-key (default: orca SSH key). Pre-staged key required.
- stepca/stepca.go: --password-file /dev/stdin removed from Init and
  issueCert. Provisioner changed to 'orca-oidc' (OIDC provisioner).
- identity/spiffe.go: --password-file removed from MintSVID. Provisioner
  changed to 'orca-oidc'.

Tests: all proxmox, stepca, identity, cli tests updated + pass. 3 new
password-rejection regression tests. Fake SSH server gains
PublicKeyCallback. go vet clean. Full build green.

internal/cli/namespace_test.go

@@ -34,7 +34,7 @@ func resetRootFlags(t *testing.T) {
 // command without resetRootFlags may call it directly.
 func resetCommandFlags() {
 	joinName, joinAddr, joinCAFinger, joinType = "", "", "", "localhost"
-	joinHost, joinSSHUser, joinPassword, proxmoxUser, proxmoxRole = "", "root", "", "orca", "OrcaOperator"
+	joinHost, joinSSHUser, joinSSHKey, proxmoxUser, proxmoxRole = "", "root", "", "orca", "OrcaOperator"
 	joinSSHPort, leaveID, nodeWatch = 22, "", false
 	stopID, runTarget, runIDKey, jobWatch = "", "", "", false
 	migrateTarget = ""