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

go-backend/internal/gateway/client.go

@@ -17,13 +17,17 @@ import (
 )
 
 // Client polls the OpenClaw gateway for agent status and keeps the database
-// and SSE broker in sync.
+// and SSE broker in sync. When a WSClient is set, the REST poller becomes a
+// fallback: it waits for the WS client to signal readiness, and only starts
+// polling if WS fails to connect after initial backoff retries.
 type Client struct {
 	url          string
 	pollInterval time.Duration
 	httpClient   *http.Client
 	agents       repository.AgentRepo
 	broker       *handler.Broker
+	wsClient     *WSClient   // optional WS client; when set, REST is fallback only
+	wsReady      chan struct{} // closed once WS connection is established
 }
 
 // Config holds gateway client configuration, typically loaded from environment.
@@ -48,22 +52,67 @@ func NewClient(cfg Config, agents repository.AgentRepo, broker *handler.Broker)
 		httpClient:   &http.Client{Timeout: 10 * time.Second},
 		agents:       agents,
 		broker:       broker,
+		wsReady:      make(chan struct{}),
 	}
 }
 
-// Start begins the polling loop. It runs until ctx is cancelled.
-func (c *Client) Start(ctx context.Context) {
-	slog.Info("gateway client starting",
-		"url", c.url,
-		"pollInterval", c.pollInterval.String())
+// SetWSClient wires the WebSocket client so the REST poller knows to defer
+// to it. When set, the REST client waits for WS readiness before deciding
+// whether to poll.
+func (c *Client) SetWSClient(ws *WSClient) {
+	c.wsClient = ws
+}
 
+// MarkWSReady signals that the WS connection is live and the REST poller
+// should stand down. Called by WSClient after a successful handshake.
+func (c *Client) MarkWSReady() {
+	select {
+	case <-c.wsReady:
+		// already closed
+	default:
+		close(c.wsReady)
+	}
+}
+
+// Start begins the gateway client loop. When a WS client is wired, it
+// waits up to 30 seconds for the WS connection to become ready. If WS
+// connects, the REST poller stands down and only logs periodically. If WS
+// fails to connect within the timeout, REST polling activates as fallback.
+func (c *Client) Start(ctx context.Context) {
+	if c.wsClient != nil {
+		slog.Info("gateway client waiting for WS connection", "timeout", "30s")
+
+		select {
+		case <-c.wsReady:
+			slog.Info("gateway client using WS — REST poller standing down")
+			// WS is live; keep this goroutine alive but idle. If WS
+			// disconnects later, we could re-enter polling, but for now
+			// the WS client handles its own reconnection.
+			<-ctx.Done()
+			slog.Info("gateway client stopped (WS mode)")
+			return
+		case <-time.After(30 * time.Second):
+			slog.Warn("gateway client: WS not ready after 30s — falling back to REST polling",
+				"url", c.url,
+				"pollInterval", c.pollInterval.String())
+		case <-ctx.Done():
+			slog.Info("gateway client stopped while waiting for WS")
+			return
+		}
+	} else {
+		slog.Info("gateway client using REST polling (no WS client configured)",
+			"url", c.url,
+			"pollInterval", c.pollInterval.String())
+	}
+
+	// REST fallback polling
 	ticker := time.NewTicker(c.pollInterval)
 	defer ticker.Stop()
 
 	for {
 		select {
 		case <-ctx.Done():
-			slog.Info("gateway client stopped")
+			slog.Info("gateway client stopped (REST fallback)")
 			return
 		case <-ticker.C:
 			c.poll(ctx)