Use ephemeral SSH keys per rebuild instead of static config keys

Generate a fresh ed25519 key pair at rebuild time, inject the public key
into the Proxmox answer file, use the private key for cluster join over
SSH, then remove the key from both the remote host and the database.
This eliminates the need to manage static SSH keys in config/secrets.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/orchestrator/sshkey.go

@@ -0,0 +1,39 @@
+package orchestrator
+
+import (
+	"crypto/ed25519"
+	"crypto/rand"
+	"encoding/pem"
+	"fmt"
+
+	"golang.org/x/crypto/ssh"
+)
+
+type KeyPair struct {
+	PrivateKey string
+	PublicKey  string
+}
+
+func GenerateEphemeralKey() (*KeyPair, error) {
+	pub, priv, err := ed25519.GenerateKey(rand.Reader)
+	if err != nil {
+		return nil, fmt.Errorf("generate ed25519 key: %w", err)
+	}
+
+	sshPub, err := ssh.NewPublicKey(pub)
+	if err != nil {
+		return nil, fmt.Errorf("ssh public key: %w", err)
+	}
+	pubStr := string(ssh.MarshalAuthorizedKey(sshPub))
+
+	privBytes, err := ssh.MarshalPrivateKey(priv, "")
+	if err != nil {
+		return nil, fmt.Errorf("marshal private key: %w", err)
+	}
+	privStr := string(pem.EncodeToMemory(privBytes))
+
+	return &KeyPair{
+		PrivateKey: privStr,
+		PublicKey:  pubStr,
+	}, nil
+}