feat(FN-1065): add immediate message response mode for heartbeats

- Add MessageResponseMode and messageResponseMode runtime config to core agent heartbeat types and exports
- Extend MessageStore with an onMessageToAgent hook plus runtime hook updates for agent-directed messages
- Wire HeartbeatMonitor to wake agents on incoming messages when messageResponseMode is immediate and state is eligible
- Update AgentDetailView with a Message Response Mode setting, validation, and runtimeConfig persistence
- Expand core and engine tests to cover hook behavior, wake-on-message triggering, and start/stop hook lifecycle

packages/core/src/message-store.ts

@@ -39,6 +39,8 @@ export interface MessageStoreEvents {
 export interface MessageStoreOptions {
   /** Root directory for kb data (default: .fusion) */
   rootDir?: string;
+  /** Optional hook invoked when a message is addressed to an agent */
+  onMessageToAgent?: (message: Message) => void;
 }
 
 /** Index structure for mailbox lookups */
@@ -55,12 +57,14 @@ export class MessageStore extends EventEmitter {
   private rootDir: string;
   private messagesDir: string;
   private indexPath: string;
+  private onMessageToAgent?: (message: Message) => void;
 
   constructor(options: MessageStoreOptions = {}) {
     super();
     this.rootDir = options.rootDir ?? ".fusion";
     this.messagesDir = join(this.rootDir, "messages");
     this.indexPath = join(this.messagesDir, "index.json");
+    this.onMessageToAgent = options.onMessageToAgent;
   }
 
   /**
@@ -109,6 +113,10 @@ export class MessageStore extends EventEmitter {
     this.emit("message:sent", message);
     this.emit("message:received", message);
 
+    if (message.toType === "agent" && this.onMessageToAgent) {
+      this.onMessageToAgent(message);
+    }
+
     return message;
   }
 
@@ -291,6 +299,13 @@ export class MessageStore extends EventEmitter {
     };
   }
 
+  /**
+   * Set or update the hook used when messages are sent to agents.
+   */
+  setMessageToAgentHook(hook: (message: Message) => void): void {
+    this.onMessageToAgent = hook;
+  }
+
   // ─────────────────────────────────────────────────────────────────────────
   // Private helpers
   // ─────────────────────────────────────────────────────────────────────────