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/opts.go

@@ -5,6 +5,7 @@ type opts struct {
 	ServerURL   string
 	GrubenvPath string
 	TimeoutSecs int
+	PubKeyPath  string
 }
 
 // parseOpts extracts command-line flags from args.
@@ -40,6 +41,11 @@ func parseOpts(args []string) opts {
 				}
 				i++
 			}
+		case "--pubkey":
+			if i+1 < len(args) {
+				o.PubKeyPath = args[i+1]
+				i++
+			}
 		}
 	}