CUB-203: fix Grimm review blocking issues (PR #41)

🔴 readLoop race: replace WriteControl close with ctx-done goroutine that closes conn
🔴 duplicate event handlers: clear handlers map before re-registering on reconnect
🔴 sync.go CurrentTask abuse: add DisplayName field to UpdateAgentRequest, use it
🔴 sync.go newRole dead code: add Role field to UpdateAgentRequest, use it
🔴 events.go handlePresence DB/SSE inconsistency: pass LastActivityAt in update, don't mutate after DB
🔴 events.go handleAgentConfig DB/SSE inconsistency: use DisplayName/Role fields in update
🟠 Send() nil-conn panic: check conn != nil before WriteJSON
🟠 readLoop prompt ctx cancellation: fixed by item #1
🟠 backoff never resets: reset to initialBackoff after successful connectAndRun
🟠 MarkWSReady double-close race: use sync.Once in Client
Extra json:"-" dead fields: removed from sessionChangedPayload, presencePayload, agentConfigPayload
UpdateAgentRequest: added DisplayName, Role, LastActivityAt fields

go-backend/internal/gateway/wsclient.go

@@ -55,6 +55,7 @@ type WSClient struct {
 	handlers     map[string][]eventHandler
 	connId       string       // set after successful hello-ok
 	restClient   *Client      // optional REST client to notify on WS ready
+	wsReadyOnce  sync.Once    // ensures MarkWSReady close is one-shot
 }
 
 // NewWSClient returns a WSClient wired to the given repository and broker.
@@ -142,8 +143,9 @@ type helloOKResponse struct {
 // read loop. On disconnect it reconnects with exponential backoff. On
 // ctx cancellation it performs a clean shutdown.
 func (c *WSClient) Start(ctx context.Context) {
-	backoff := 1 * time.Second
+	initialBackoff := 1 * time.Second
 	maxBackoff := 30 * time.Second
+	backoff := initialBackoff
 
 	for {
 		err := c.connectAndRun(ctx)
@@ -155,6 +157,9 @@ func (c *WSClient) Start(ctx context.Context) {
 			c.logger.Warn("ws client disconnected, reconnecting",
 				"error", err,
 				"backoff", backoff)
+		} else {
+			// Reset backoff on successful connect+run completion
+			backoff = initialBackoff
 		}
 
 		select {
@@ -189,7 +194,16 @@ func (c *WSClient) connectAndRun(ctx context.Context) error {
 	c.conn = conn
 	c.connMu.Unlock()
 
-	// Reset backoff on successful connect
+	// When context is cancelled, close the conn to unblock ReadJSON in readLoop.
+	go func() {
+		<-ctx.Done()
+		c.connMu.Lock()
+		if c.conn != nil {
+			c.conn.Close()
+		}
+		c.connMu.Unlock()
+	}()
+
 	defer func() {
 		conn.Close()
 	}()
@@ -221,6 +235,9 @@ func (c *WSClient) connectAndRun(ctx context.Context) error {
 		c.logger.Info("ws client notified REST fallback to stand down")
 	}
 
+	// Reset wsReadyOnce so MarkWSReady can fire again after a reconnect
+	c.wsReadyOnce = sync.Once{}
+
 	// 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)
@@ -309,25 +326,15 @@ func (c *WSClient) sendConnect(conn *websocket.Conn) (*helloOKResponse, error) {
 }
 
 // readLoop continuously reads frames from the connection and routes them.
-// It returns on read error or context cancellation.
+// It returns on read error or when the connection is closed by the ctx-done
+// goroutine started in connectAndRun.
 func (c *WSClient) readLoop(ctx context.Context, conn *websocket.Conn) error {
 	for {
-		select {
-		case <-ctx.Done():
-			// Clean shutdown: send close frame
-			c.connMu.Lock()
-			c.conn.WriteControl(
-				websocket.CloseMessage,
-				websocket.FormatCloseMessage(websocket.CloseNormalClosure, "shutdown"),
-				time.Now().Add(5*time.Second),
-			)
-			c.connMu.Unlock()
-			return ctx.Err()
-		default:
-		}
-
 		var frame wsFrame
 		if err := conn.ReadJSON(&frame); err != nil {
+			if ctx.Err() != nil {
+				return ctx.Err()
+			}
 			// Check if it's a close error
 			if websocket.IsCloseError(err, websocket.CloseNormalClosure, websocket.CloseGoingAway) {
 				c.logger.Info("ws connection closed by server")
@@ -398,9 +405,8 @@ func (c *WSClient) handleEvent(frame wsFrame) {
 // ── Send ─────────────────────────────────────────────────────────────────
 
 // Send sends a JSON request to the gateway and returns the response payload.
-// It is safe for concurrent use. The caller should check for errors in the
-// returned payload. A nil payload with nil error means the gateway sent an
-// error response (check via the response frame's error field, which is logged).
+// It is safe for concurrent use. Returns an error if the client is not
+// connected.
 func (c *WSClient) Send(method string, params any) (json.RawMessage, error) {
 	reqID := uuid.New().String()
 
@@ -430,6 +436,10 @@ func (c *WSClient) Send(method string, params any) (json.RawMessage, error) {
 	}
 
 	c.connMu.Lock()
+	if c.conn == nil {
+		c.connMu.Unlock()
+		return nil, fmt.Errorf("gateway: not connected")
+	}
 	err = c.conn.WriteJSON(frame)
 	c.connMu.Unlock()