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/sync.go

@@ -42,6 +42,11 @@ type sessionListItem struct {
 // persists them, merges session state into agent cards, and broadcasts
 // the merged fleet as a fleet.update event.
 func (c *WSClient) initialSync(ctx context.Context) error {
+	if c.agents == nil {
+		c.logger.Info("initial sync skipped (no repository)")
+		return nil
+	}
+
 	c.logger.Info("initial sync starting")
 
 	// 1. Fetch agents
@@ -77,12 +82,12 @@ func (c *WSClient) initialSync(ctx context.Context) error {
 			newName := card.DisplayName
 			newRole := card.Role
 			_, updateErr := c.agents.Update(ctx, card.ID, models.UpdateAgentRequest{
-				CurrentTask: &newName, // reuse field for display name update
+				DisplayName: &newName,
+				Role:        &newRole,
 			})
 			if updateErr != nil {
 				c.logger.Warn("sync: agent update failed", "id", card.ID, "error", updateErr)
 			}
-			_ = newRole // role not in UpdateAgentRequest yet, skip silently
 		}
 	}