CUB-200: implement WebSocket gateway client with v3 protocol

Replace REST poller with WebSocket client as primary gateway connection:

- wsclient.go: WebSocket client with v3 handshake (connect.challenge →
  connect → hello-ok), frame routing (req/res/event), JSON-RPC Send(),
  auto-reconnect with exponential backoff (1s → 30s max)
- sync.go: Initial sync via agents.list + sessions.list RPCs, merge
  session runtime state into AgentCardData, broadcast fleet.update
- events.go: Real-time event handlers for sessions.changed, presence,
  and agent.config — DB update first, then SSE broadcast
- client.go: REST poller retained as fallback (WS is primary)
- config.go: Add GATEWAY_WS_URL and OPENCLAW_GATEWAY_TOKEN env vars
- main.go: Wire WS client as primary, REST as fallback
- .env.example: Document new WS config vars

Fallback: If WS connection fails, seeded demo data + REST polling
remain available.

go-backend/cmd/server/main.go

@@ -63,16 +63,29 @@ func main() {
 		Broker:     broker,
 	})
 
-	// ── Gateway client (polls OpenClaw for agent states) ───────────────────
-	gwClient := gateway.NewClient(gateway.Config{
+	// ── Gateway: WS primary + REST fallback ────────────────────────────────
+	// WebSocket client (primary — real-time events via OpenClaw v3 protocol)
+	wsClient := gateway.NewWSClient(gateway.WSConfig{
+		URL:       cfg.WSGatewayURL,
+		AuthToken: cfg.WSGatewayToken,
+	}, agentRepo, broker, logger)
+
+	// REST polling client (fallback — only used if WS connection fails)
+	restClient := gateway.NewClient(gateway.Config{
 		URL:          cfg.GatewayURL,
 		PollInterval: cfg.GatewayPollInterval,
 	}, agentRepo, broker)
 
+	// Wire them: WS notifies REST to stand down on successful connect
+	wsClient.SetRESTClient(restClient)
+
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 
-	go gwClient.Start(ctx)
+	// Start WS client first (primary)
+	go wsClient.Start(ctx)
+	// Start REST client (fallback polling)
+	go restClient.Start(ctx)
 
 	// ── Server ─────────────────────────────────────────────────────────────
 	srv := &http.Server{
@@ -98,7 +111,7 @@ func main() {
 	<-quit
 	slog.Info("shutting down server...")
 
-	cancel() // stop gateway polling
+	cancel() // stop gateway clients
 
 	shutdownCtx, shutdownCancel := context.WithTimeout(context.Background(), 15*time.Second)
 	defer shutdownCancel()
@@ -122,4 +135,4 @@ func parseLogLevel(level string) slog.Level {
 	default:
 		return slog.LevelInfo
 	}
-}
+}