fix(agent-engines): runnable Docker image, reachable WebSocket server, remove stale build/

[feyishola]

Summary

Closes #1421, closes #1424, closes #1425, closes #1426

#1421: WebSocket server bound to localhost

• main.py reads AGENT_WS_HOST and AGENT_WS_PORT (default 0.0.0.0:8765). The Dockerfile has EXPOSE 8765.
• shutdown() now closes the WebSocket server.

#1424: Legacy (websocket, path) handler

• The handler takes one argument and reads the path from websocket.request.path.
• websockets was not declared as a dependency. It is now pinned to >=13.0,<16.

#1425: Docker image couldn't run

• The builder copies pyproject.toml, README.md and gpu_worker/ before pip install, and installs into /opt/venv, owned by appuser.
• The runtime image installs Stockfish and sets STOCKFISH_PATH=/usr/games/stockfish and ENGINE_BACKEND=stockfish.
• Added .dockerignore.
• New docker-build CI job:
  • builds the image
  • runs import gpu_worker
  • checks that the Stockfish binary responds to uci
  • does a real WebSocket round-trip into the running container. A plain TCP connect isn't enough because docker-proxy accepts connections even when the app has crashed.

#1426: Stale build/ directory

• Removed the 18 tracked files under agent-engines/build/.
• Added build/, dist/ and *.egg-info/ to agent-engines/.gitignore. I scoped this to agent-engines/ so the rules don't affect other packages in the monorepo.

Two other fixes needed for the container to start

• main() registered signal handlers with getattr(asyncio, 'SIGINT'), which doesn't exist, so the process crashed at startup. It now uses the signal module.
• The default worker backend is lc0, which isn't installed in the image, so the container exited at startup. The backend can now be chosen with ENGINE_BACKEND. Without it, the default is still lc0.

Test plan

• docker build succeeds. import gpu_worker works, websockets is 15.0.1, and Stockfish 17.1 responds to uci.
• Ran the container with -p. A client on the host sent messages and got the expected replies, and the server logged the request path.
• docker stop (SIGTERM) shuts down cleanly.
• pytest tests: 503 passed and 6 skipped, the same as main. test_orchestrator.py: 3 passed.

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