feat: add production hardening — Ed25519 signing, Portainer Edge, SSH extension (Phase 4)

Image signing:
- Ed25519 sign/verify package (pure Go stdlib, zero deps)
- genkey and sign CLI subcommands for build system
- Optional --pubkey flag for verifying updates on apply
- Signature URLs in update metadata (latest.json)

Portainer Edge Agent:
- cloud-init portainer.go module writes K8s manifest
- Auto-deploys Edge Agent when portainer.edge-agent.enabled
- Full RBAC (ServiceAccount, ClusterRoleBinding, Deployment)
- 5 Portainer tests in portainer_test.go

Production tooling:
- SSH debug extension builder (hack/build-ssh-extension.sh)
- Boot performance benchmark (test/benchmark/bench-boot.sh)
- Resource usage benchmark (test/benchmark/bench-resources.sh)
- Deployment guide (docs/deployment-guide.md)

Test results: 50 update agent tests + 22 cloud-init tests passing.

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

update/cmd/apply.go

@@ -33,6 +33,12 @@ func Apply(args []string) error {
 	client := image.NewClient(opts.ServerURL, stageDir)
 	defer client.Cleanup()
 
+	// Enable signature verification if public key is configured
+	if opts.PubKeyPath != "" {
+		client.SetPublicKeyPath(opts.PubKeyPath)
+		slog.Info("signature verification enabled", "pubkey", opts.PubKeyPath)
+	}
+
 	meta, err := client.CheckForUpdate()
 	if err != nil {
 		return fmt.Errorf("checking for update: %w", err)