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/wsclient.go

@@ -43,17 +43,18 @@ type eventHandler func(json.RawMessage)
 // v3 handshake, routes incoming frames, and automatically reconnects on
 // disconnect with exponential backoff.
 type WSClient struct {
-	config  WSConfig
-	conn    *websocket.Conn
-	connMu  sync.Mutex // protects conn for writes
-	pending map[string]chan<- json.RawMessage
-	mu      sync.Mutex // protects pending and handlers
-	agents  repository.AgentRepo
-	broker  *handler.Broker
-	logger  *slog.Logger
+	config       WSConfig
+	conn         *websocket.Conn
+	connMu       sync.Mutex // protects conn for writes
+	pending      map[string]chan<- json.RawMessage
+	mu           sync.Mutex // protects pending and handlers
+	agents       repository.AgentRepo
+	broker       *handler.Broker
+	logger       *slog.Logger
 
-	handlers map[string][]eventHandler
-	connId   string // set after successful hello-ok
+	handlers     map[string][]eventHandler
+	connId       string       // set after successful hello-ok
+	restClient   *Client      // optional REST client to notify on WS ready
 }
 
 // NewWSClient returns a WSClient wired to the given repository and broker.
@@ -71,6 +72,12 @@ func NewWSClient(cfg WSConfig, agents repository.AgentRepo, broker *handler.Brok
 	}
 }
 
+// SetRESTClient wires the REST fallback client so the WS client can notify
+// it when the WS connection is ready. Call this before Start.
+func (c *WSClient) SetRESTClient(rest *Client) {
+	c.restClient = rest
+}
+
 // OnEvent registers a handler for the given event name. Handlers are called
 // when an incoming frame with type "event" and matching event name is
 // received. This is safe to call before Start.
@@ -208,6 +215,12 @@ func (c *WSClient) connectAndRun(ctx context.Context) error {
 	c.connId = helloOK.ConnID
 	c.connMu.Unlock()
 
+	// Notify REST client that WS is live so it stands down
+	if c.restClient != nil {
+		c.restClient.MarkWSReady()
+		c.logger.Info("ws client notified REST fallback to stand down")
+	}
+
 	// Step 2b: Initial sync — fetch agents + sessions from gateway
 	if err := c.initialSync(ctx); err != nil {
 		c.logger.Warn("initial sync failed, will continue with read loop", "error", err)