refactor(ws/heartbeat): accept agent_key or UUID via cache-aware resolver

All 7 heartbeat handlers (get/set/toggle/test/logs/checklistGet/checklistSet)
now accept either a UUID or an agent_key for params.agentId. Behavior
additive — existing UUID-only clients keep working.

Adds HeartbeatMethods.SetAgentRouter setter wired from cmd/gateway_methods.go
so the fast path avoids a DB roundtrip for cached agents. Preserves the
literal "invalid agentId" error string (no i18n key change).

Phase 3 of agent identity hardening.

cmd/gateway_methods.go

@@ -43,10 +43,15 @@ func registerAllMethods(server *gateway.Server, agents *agent.Router, sessStore
 
 	// Phase 2: Heartbeat
 	heartbeatMethods := methods.NewHeartbeatMethods(heartbeatStore, msgBus)
+	// Wire cache-aware resolver (Phase 3): accepts agent_key or UUID input
+	// without a DB roundtrip on the hot path when the agent is router-cached.
+	heartbeatMethods.SetAgentRouter(agents)
 	heartbeatMethods.Register(router)
 
 	// Phase 2: Config permissions
-	methods.NewConfigPermissionsMethods(configPermStore, agentStore).Register(router)
+	cfgPerms := methods.NewConfigPermissionsMethods(configPermStore, agentStore)
+	cfgPerms.SetAgentRouter(agents)
+	cfgPerms.Register(router)
 
 	// Phase 2: Pairing (store created externally, shared with channel manager).
 	// OnApprove callback is set later by the caller after channel manager is created.