fix: session stability improvements

- Fix docker client initialization bug in app.py (context manager was closing client)
- Add restart_session() method to preserve session IDs during container restarts
- Add 60-second startup grace period before health checking new sessions
- Fix _stop_container and _get_container_info to use docker_service API consistently
- Disable mDNS in Dockerfile to prevent Bonjour service name conflicts
- Remove old container before restart to free port bindings

Dockerfile

@@ -28,5 +28,5 @@ EXPOSE 8080
 # Set environment variables
 ENV PYTHONPATH=/app
 
-# Start OpenCode server (OPENCODE_API_KEY passed via environment)
-CMD ["/bin/bash", "-c", "source /root/.bashrc && opencode serve --hostname 0.0.0.0 --port 8080 --mdns"]
+# Start OpenCode server (mDNS disabled to prevent conflicts between containers)
+CMD ["/bin/bash", "-c", "source /root/.bashrc && opencode serve --hostname 0.0.0.0 --port 8080"]