Add server-initiated restart and self-update capabilities for remote
agents. Restart broadcasts on the lifecycle PubSub topic causing the
agent to exit cleanly for Docker to restart. Self-update fetches the
latest release from GitHub, finds the matching binary for the agent's
architecture, and sends the download URL with SHA256 checksum.
- Add restart_agent/1 and update_agent/3 to Agents context
- Handle :restart_requested and :update_requested in AgentChannel
- Add superuser-only Restart and Update buttons on agent show page
- Add ReleaseChecker module for GitHub Releases API with 5-min cache
- Add arch field to AgentHeartbeat protobuf for architecture detection
- Store arch in agent metadata from heartbeat