fix(announce): use deterministic idempotency keys to prevent duplicate subagent announces (#17150)

Merged via /review-pr -> /prepare-pr -> /merge-pr.

Prepared head SHA: 54bba3cea1
Co-authored-by: widingmarcus-cyber <245375637+widingmarcus-cyber@users.noreply.github.com>
Co-authored-by: gumadeiras <5599352+gumadeiras@users.noreply.github.com>
Reviewed-by: @gumadeiras

src/agents/subagent-announce-queue.ts

@@ -14,6 +14,9 @@ import {
 } from "../utils/queue-helpers.js";
 
 export type AnnounceQueueItem = {
+  // Stable announce identity shared by direct + queued delivery paths.
+  // Optional for backward compatibility with previously queued items.
+  announceId?: string;
   prompt: string;
   summaryLine?: string;
   enqueuedAt: number;