CUB-204: wire WS client as primary, REST poller as fallback

- Rename GatewayURL/GatewayPollInterval → GatewayRestURL/GatewayRestPollInterval
- Change Docker-aware defaults (host.docker.internal instead of localhost)
- Client.Start() waits for WS readiness (30s timeout), falls back to REST
- Client.SetWSClient()/MarkWSReady() for WS→REST coordination
- WSClient.SetRESTClient() so WS notifies REST on successful handshake
- main.go wires both clients: WS primary, REST fallback with cross-references
- .env.example documents WS_GATEWAY_URL, GATEWAY_TOKEN, REST fallback vars
- docker-compose.yml adds WS_GATEWAY_URL and GATEWAY_TOKEN env vars
- reference/CONTROL_CENTER_CONTEXT.md documents architecture and startup sequence

docker-compose.yml

@@ -16,6 +16,8 @@ services:
       - ENVIRONMENT=production
       - PORT=8080
       - GATEWAY_URL=http://host.docker.internal:18789/api/agents
+      - WS_GATEWAY_URL=ws://host.docker.internal:18789/
+      - GATEWAY_TOKEN=${GATEWAY_TOKEN:-}
     depends_on:
       db:
         condition: service_healthy