feat: add timestamps to session messages

src/session/manager.ts

@@ -15,9 +15,14 @@ export class ManagedSession implements Session {
     private history: Message[] = []
   ) {}
 
-  addMessage(message: Message): void {
-    this.history.push(message);
-    this.store.addMessage(this.id, message);
+  addMessage(message: Message): Message {
+    const messageWithTimestamp: Message = {
+      ...message,
+      timestamp: Date.now(),
+    };
+    this.history.push(messageWithTimestamp);
+    this.store.addMessage(this.id, messageWithTimestamp);
+    return messageWithTimestamp;
   }
 
   getHistory(): Message[] {