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/announce-idempotency.ts

@@ -0,0 +1,25 @@
+export type AnnounceIdFromChildRunParams = {
+  childSessionKey: string;
+  childRunId: string;
+};
+
+export function buildAnnounceIdFromChildRun(params: AnnounceIdFromChildRunParams): string {
+  return `v1:${params.childSessionKey}:${params.childRunId}`;
+}
+
+export function buildAnnounceIdempotencyKey(announceId: string): string {
+  return `announce:${announceId}`;
+}
+
+export function resolveQueueAnnounceId(params: {
+  announceId?: string;
+  sessionKey: string;
+  enqueuedAt: number;
+}): string {
+  const announceId = params.announceId?.trim();
+  if (announceId) {
+    return announceId;
+  }
+  // Backward-compatible fallback for queue items that predate announceId.
+  return `legacy:${params.sessionKey}:${params.enqueuedAt}`;
+}