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

.env.example

@@ -13,9 +13,14 @@ ENVIRONMENT=development
 DATABASE_URL=postgresql://controlcenter:controlcenter@localhost:5432/controlcenter?sslmode=disable
 
 # Gateway (OpenClaw) connection
-# URL to the OpenClaw gateway API for polling agent states
-GATEWAY_URL=http://localhost:18789/api/agents
-# Polling interval for agent state updates
+# WebSocket gateway config (primary path)
+WS_GATEWAY_URL=ws://host.docker.internal:18789/
+# Gateway auth token — same as OPENCLAW_GATEWAY_TOKEN (set in environment)
+GATEWAY_TOKEN=
+
+# REST poller config (fallback, only used if WS fails to connect)
+GATEWAY_URL=http://host.docker.internal:18789/api/agents
+# Polling interval for agent state updates (fallback only)
 GATEWAY_POLL_INTERVAL=5s
 
 # ── Frontend Variables (via Vite) ───────────────────────────────────────