chore(agents): remove dead UUID invalidation in agents_update

Post-Phase-2 canonicalization, router cache entries are always keyed as
tenantID:agent_key. The belt-and-suspenders m.agents.InvalidateAgent(ag.ID.String())
call never matched any entry — exact-segment match on the final ":"-
delimited segment cannot match a UUID as the agentKey.

Removes the dead call and the misleading comment that justified it.

Phase 2 follow-up (H7).

internal/gateway/methods/agents_update.go

@@ -237,10 +237,11 @@ func (m *AgentsMethods) handleUpdate(ctx context.Context, client *gateway.Client
 			}
 		}
 
+		// Post-Phase-2 canonicalization: router cache entries are always keyed
+		// as `tenantID:agent_key`. The previous belt-and-suspenders UUID-based
+		// invalidation was dead code — exact-segment match never matches a
+		// UUID as the final segment of a canonical cache key.
 		m.agents.InvalidateAgent(params.AgentID)
-		// Also invalidate by UUID — heartbeat/cron sessions cached under UUID key
-		// before the agentKey fix may still be in the router cache.
-		m.agents.InvalidateAgent(ag.ID.String())
 	}
 
 	client.SendResponse(protocol.NewOKResponse(req.ID, map[string]any{