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/client.go

@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"log/slog"
 	"net/http"
+	"sync"
 	"time"
 
 	"code.cubecraftcreations.com/CubeCraft-Creations/Control-Center/go-backend/internal/handler"
@@ -26,8 +27,9 @@ type Client struct {
 	httpClient   *http.Client
 	agents       repository.AgentRepo
 	broker       *handler.Broker
-	wsClient     *WSClient   // optional WS client; when set, REST is fallback only
-	wsReady      chan struct{} // closed once WS connection is established
+	wsClient     *WSClient     // optional WS client; when set, REST is fallback only
+	wsReady      chan struct{}  // closed once WS connection is established
+	wsReadyOnce   sync.Once     // protects wsReady close from double-close race
 }
 
 // Config holds gateway client configuration, typically loaded from environment.
@@ -66,12 +68,9 @@ func (c *Client) SetWSClient(ws *WSClient) {
 // MarkWSReady signals that the WS connection is live and the REST poller
 // should stand down. Called by WSClient after a successful handshake.
 func (c *Client) MarkWSReady() {
-	select {
-	case <-c.wsReady:
-		// already closed
-	default:
+	c.wsReadyOnce.Do(func() {
 		close(c.wsReady)
-	}
+	})
 }
 
 // Start begins the gateway client loop. When a WS client is wired, it