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/project-engine.ts

@@ -268,6 +268,11 @@ export class ProjectEngine {
     return this.runtime.getAgentStore();
   }
 
+  /** Get the MessageStore (if initialized). Returns undefined before start(). */
+  getMessageStore(): import("@fusion/core").MessageStore | undefined {
+    return this.runtime.getMessageStore();
+  }
+
   /** Get the HeartbeatMonitor (if initialized). */
   getHeartbeatMonitor() {
     return this.runtime.getHeartbeatMonitor();