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/config/config.go

@@ -15,10 +15,10 @@ type Config struct {
 	CORSOrigin          string
 	LogLevel            string
 	Environment         string
-	GatewayURL          string
-	GatewayPollInterval time.Duration
-	WSGatewayURL        string
-	WSGatewayToken      string
+	GatewayRestURL          string
+	GatewayRestPollInterval time.Duration
+	WSGatewayURL            string
+	WSGatewayToken          string
 }
 
 // Load reads configuration from environment variables, applying defaults where
@@ -30,10 +30,10 @@ func Load() *Config {
 		CORSOrigin:          getEnv("CORS_ORIGIN", "*"),
 		LogLevel:            getEnv("LOG_LEVEL", "info"),
 		Environment:         getEnv("ENVIRONMENT", "development"),
-		GatewayURL:          getEnv("GATEWAY_URL", "http://localhost:18789/api/agents"),
-		GatewayPollInterval: getEnvDuration("GATEWAY_POLL_INTERVAL", 5*time.Second),
-		WSGatewayURL:        getEnv("WS_GATEWAY_URL", "ws://localhost:18789/"),
-		WSGatewayToken:      getEnv("OPENCLAW_GATEWAY_TOKEN", ""),
+		GatewayRestURL:          getEnv("GATEWAY_URL", "http://host.docker.internal:18789/api/agents"),
+		GatewayRestPollInterval: getEnvDuration("GATEWAY_POLL_INTERVAL", 5*time.Second),
+		WSGatewayURL:            getEnv("WS_GATEWAY_URL", "ws://host.docker.internal:18789/"),
+		WSGatewayToken:          getEnv("OPENCLAW_GATEWAY_TOKEN", ""),
 	}
 }