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/internal/config/config.go

@@ -10,13 +10,15 @@ import (
 
 // Config holds all application configuration.
 type Config struct {
-	Port              int
-	DatabaseURL       string
-	CORSOrigin        string
-	LogLevel          string
-	Environment       string
-	GatewayURL        string
-	GatewayPollInterval time.Duration
+	Port                    int
+	DatabaseURL             string
+	CORSOrigin              string
+	LogLevel                string
+	Environment            string
+	GatewayURL              string        // REST fallback URL
+	GatewayPollInterval     time.Duration // REST fallback poll interval
+	WSGatewayURL            string        // WebSocket gateway URL
+	WSGatewayToken          string        // WebSocket auth token
 }
 
 // Load reads configuration from environment variables, applying defaults where
@@ -28,8 +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"),
+		GatewayURL:          getEnv("GATEWAY_URL", "http://host.docker.internal:18789/api/agents"),
 		GatewayPollInterval: getEnvDuration("GATEWAY_POLL_INTERVAL", 5*time.Second),
+		WSGatewayURL:        getEnv("GATEWAY_WS_URL", "ws://host.docker.internal:18789/"),
+		WSGatewayToken:      getEnv("OPENCLAW_GATEWAY_TOKEN", ""),
 	}
 }
 
@@ -56,4 +60,4 @@ func getEnvDuration(key string, fallback time.Duration) time.Duration {
 		}
 	}
 	return fallback
-}
+}