feat(FN-1976): forward mailbox message events end to end

- Wire MessageStore into the in-process executor runtime and project engine
- Forward MessageStore events through dashboard SSE infrastructure for mailbox updates
- Close mailbox pipeline gaps across API routes, server wiring, and mailbox UI components
- Add regression coverage for messaging routes, SSE forwarding, and agent tool behavior
- Document the MessageStore SSE wiring pattern in .fusion/memory.md

packages/engine/src/runtimes/in-process-runtime.ts

@@ -131,7 +131,12 @@ export class InProcessRuntime
 
     try {
       // 1. Initialize TaskStore (use external if provided, otherwise create new)
-      const { TaskStore, PluginStore: PluginStoreClass, PluginLoader: PluginLoaderClass } = await import("@fusion/core");
+      const {
+        TaskStore,
+        PluginStore: PluginStoreClass,
+        PluginLoader: PluginLoaderClass,
+        MessageStore: MessageStoreClass,
+      } = await import("@fusion/core");
       if (this.config.externalTaskStore) {
         this.taskStore = this.config.externalTaskStore;
         runtimeLog.log(`TaskStore provided externally for project ${this.config.projectId}`);
@@ -141,6 +146,9 @@ export class InProcessRuntime
         runtimeLog.log(`TaskStore initialized for project ${this.config.projectId}`);
       }
 
+      // Initialize MessageStore early so TaskExecutor receives send_message capability.
+      this.messageStore = new MessageStoreClass(this.taskStore.getDatabase());
+
       // 2. Initialize Plugin system (PluginStore + PluginLoader + PluginRunner)
       this.pluginStore = new PluginStoreClass(this.taskStore.getFusionDir());
       await this.pluginStore.init();
@@ -263,6 +271,7 @@ export class InProcessRuntime
         usageLimitPauser: this.usageLimitPauser,
         stuckTaskDetector: this.stuckTaskDetector,
         pluginRunner: this.pluginRunner,
+        messageStore: this.messageStore,
         missionStore,
         onSliceComplete: (slice) => {
           void this.scheduler.onSliceComplete(slice);
@@ -353,13 +362,10 @@ export class InProcessRuntime
 
       // 6. Initialize AgentStore and HeartbeatMonitor
       try {
-        const { AgentStore: AgentStoreClass, MessageStore: MessageStoreClass } = await import("@fusion/core");
+        const { AgentStore: AgentStoreClass } = await import("@fusion/core");
         this.agentStore = new AgentStoreClass({ rootDir: this.taskStore.getFusionDir() });
         await this.agentStore.init();
 
-        // Initialize MessageStore for wake-on-message behavior
-        this.messageStore = new MessageStoreClass(this.taskStore.getDatabase());
-
         this.heartbeatMonitor = new HeartbeatMonitor({
           store: this.agentStore,
           agentStore: this.agentStore, // enables per-agent config resolution
@@ -718,6 +724,14 @@ export class InProcessRuntime
     return this.agentStore;
   }
 
+  /**
+   * Get the MessageStore instance (if initialized).
+   * Returns undefined before start() or if initialization fails.
+   */
+  getMessageStore(): import("@fusion/core").MessageStore | undefined {
+    return this.messageStore;
+  }
+
   /**
    * Get the project's Scheduler instance.
    * @throws Error if runtime has not been started