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/identity/spiffe.go

@@ -11,14 +11,14 @@ import (
 )
 
 var (
-	ErrStepCLI         = errors.New("identity: step CLI failed")
+	ErrStepCLI          = errors.New("identity: step CLI failed")
 	ErrSpiffeURIMissing = errors.New("identity: spiffe URI SAN missing")
 )
 
 const (
-	SpiffeTrustDomain   = "orca.local"
-	SVIDNotAfter        = "24h"
-	DefaultProvisioner  = "orca-admin"
+	SpiffeTrustDomain  = "orca.local"
+	SVIDNotAfter       = "24h"
+	DefaultProvisioner = "orca-admin"
 )
 
 type execer interface {
@@ -51,8 +51,8 @@ func MintSVID(ctx context.Context, transport execer, leadPeer, namespace, sa, al
 	sb.WriteString(" --not-after ")
 	sb.WriteString(shellQuote(SVIDNotAfter))
 	sb.WriteString(" --provisioner ")
-	sb.WriteString(shellQuote(DefaultProvisioner))
-	sb.WriteString(" --password-file /dev/stdin --force")
+	sb.WriteString(shellQuote("orca-oidc"))
+	sb.WriteString(" --force")
 	cmd := sb.String()
 	if _, err := transport.Exec(ctx, leadPeer, cmd); err != nil {
 		return nil, nil, fmt.Errorf("identity: mint %s: %w", spiffeID, err)