feat(FN-4583): complete Step 2 — expose engine chat store accessor

Fusion-Task-Id: FN-4583
Fusion-Task-Lineage: 4d9b088d-a705-4376-b547-15b37e5370aa

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

@@ -1004,6 +1004,14 @@ export class InProcessRuntime
     return this.messageStore;
   }
 
+  /**
+   * Get the ChatStore instance (if initialized).
+   * Returns undefined before start() or if initialization fails.
+   */
+  getChatStore(): import("@fusion/core").ChatStore | undefined {
+    return this.chatStore;
+  }
+
   /**
    * Get the project's Scheduler instance.
    * @throws Error if runtime has not been started