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/daemon_test.go

@@ -224,7 +224,7 @@ func TestNodeJoinProxmoxNoMTLSDeprecationWarning(t *testing.T) {
 	rootCmd.SetErr(&out)
 	// proxmox path errors on missing --host before reaching the warning,
 	// and never calls joinLocal, so no mTLS deprecation warning fires.
-	rootCmd.SetArgs([]string{"node", "join", "--type", "proxmox", "--password", "x"})
+	rootCmd.SetArgs([]string{"node", "join", "--type", "proxmox", "--ssh-key", "/tmp/nonexistent-key"})
 	_ = rootCmd.Execute()
 
 	if strings.Contains(buf.String(), "mTLS join path is deprecated") {