fix: prefer requesterOrigin over stale session entry in subagent announce routing (#4957)

* fix: prefer requesterOrigin over stale session entry in subagent announce routing

When a subagent finishes and announces results back, resolveAnnounceOrigin
merged the session entry (primary) with requesterOrigin (fallback). If the
session store had a stale lastChannel (e.g. whatsapp) from a previous
interaction but the user was now on a different channel (e.g. bluebubbles),
the announce would route to the wrong channel.

Swap the merge order so requesterOrigin (captured at spawn time, reflecting
the actual current channel) takes priority, with the session entry as
fallback for any missing fields.

Error before fix:
  Delivery failed (whatsapp to bluebubbles:chat_guid:...): Unknown channel: whatsapp

Adds regression test for the stale-channel scenario.

* fix: match test to exact failure scenario and improve reliability (#4957) (thanks @tyler6204)

- Remove lastTo from stale session store to match the exact mismatch scenario described in the PR
- Replace 5ms setTimeout sleeps with expect.poll for better test reliability
- Prevents flakiness on slower CI machines

src/agents/subagent-announce.ts

@@ -93,7 +93,10 @@ function resolveAnnounceOrigin(
   entry?: DeliveryContextSource,
   requesterOrigin?: DeliveryContext,
 ): DeliveryContext | undefined {
-  return mergeDeliveryContext(deliveryContextFromSession(entry), requesterOrigin);
+  // requesterOrigin (captured at spawn time) reflects the channel the user is
+  // actually on and must take priority over the session entry, which may carry
+  // stale lastChannel / lastTo values from a previous channel interaction.
+  return mergeDeliveryContext(requesterOrigin, deliveryContextFromSession(entry));
 }
 
 async function sendAnnounce(item: AnnounceQueueItem) {