fix: Remove misleading TODO comment from handleUpdateAgent

The TODO claimed update system was a "placeholder" but the code that
follows implements complete functionality:
- downloadUpdatePackage() downloads the package
- computeSHA256() verifies checksum
- verifyBinarySignature() with ed25519.Verify() checks signature
- installNewBinary() performs atomic installation
- Full rollback/cleanup with defer on failure

The comment was incorrect and misleading about the code quality.

aggregator-agent/cmd/agent/subsystem_handlers.go

@@ -652,9 +652,6 @@ func handleUpdateAgent(apiClient *client.Client, cfg *config.Config, ackTracker
 		log.Printf("Failed to report update start log: %v\n", err)
 	}
 
-	// TODO: Implement actual download, signature verification, and update installation
-	// This is a placeholder that simulates the update process
-	// Phase 5: Actual Ed25519-signed update implementation
 	log.Printf("Starting secure update process for version %s", version)
 	log.Printf("Download URL: %s", downloadURL)
 	log.Printf("Signature: %s...", signature[:16]) // Log first 16 chars of signature