Handle server-initiated restart (exit for Docker restart) and
self-update (download binary, verify SHA256, replace via exec)
commands received over the WebSocket channel. Report architecture
in heartbeat so the server can select the correct binary.
- Add restart and update handlers in websocket_client.rs
- Add self_update() with download, checksum verification, and exec
- Add arch field to AgentHeartbeat protobuf
- Add reqwest and sha2 dependencies
- chown binary in Dockerfile for non-root self-update
- Extract standalone binaries from Docker images in CI
- Create GitHub Releases with binary assets on version tags