feat: add background task notification system (#827)

Co-authored-by: Letta <noreply@letta.com>

src/cli/helpers/messageQueueBridge.ts

@@ -0,0 +1,63 @@
+/**
+ * Message Queue Bridge
+ *
+ * Allows non-React code (like Task.ts) to add messages to the messageQueue.
+ * The queue adder function is set by App.tsx on mount.
+ *
+ * This enables background tasks to queue their notification XML directly
+ * into messageQueue, where the existing dequeue logic handles auto-firing.
+ */
+
+export type QueuedMessage = {
+  kind: "user" | "task_notification";
+  text: string;
+};
+
+type QueueAdder = (message: QueuedMessage) => void;
+
+let queueAdder: QueueAdder | null = null;
+const pendingMessages: QueuedMessage[] = [];
+const MAX_PENDING_MESSAGES = 10;
+
+/**
+ * Set the queue adder function. Called by App.tsx on mount.
+ */
+export function setMessageQueueAdder(fn: QueueAdder | null): void {
+  queueAdder = fn;
+  if (queueAdder && pendingMessages.length > 0) {
+    for (const message of pendingMessages) {
+      queueAdder(message);
+    }
+    pendingMessages.length = 0;
+  }
+}
+
+/**
+ * Add a message to the messageQueue.
+ * Called from Task.ts when a background task completes.
+ * If queue adder not set (App not mounted), message is dropped.
+ */
+export function addToMessageQueue(message: QueuedMessage): void {
+  if (queueAdder) {
+    queueAdder(message);
+    return;
+  }
+  if (pendingMessages.length >= MAX_PENDING_MESSAGES) {
+    pendingMessages.shift();
+  }
+  pendingMessages.push(message);
+}
+
+/**
+ * Check if the queue bridge is connected.
+ */
+export function isQueueBridgeConnected(): boolean {
+  return queueAdder !== null;
+}
+
+/**
+ * Clear any pending messages (for testing).
+ */
+export function clearPendingMessages(): void {
+  pendingMessages.length = 0;
+}