fix: clear delivery routing state when creating isolated cron sessions (#27778)

* fix: clear delivery routing state when creating isolated cron sessions

When `resolveCronSession()` creates a new session (forceNew / isolated),
the `...entry` spread preserves `lastThreadId`, `lastTo`, `lastChannel`,
and `lastAccountId` from the prior session. This causes announce-mode
cron deliveries to post as thread replies instead of channel top-level
messages when `delivery.to` matches the channel of a prior conversation.

Clear delivery routing metadata on new session creation so isolated
cron sessions start with a clean delivery state.

Closes #27751

✍️ Author: Claude Code with @carrotRakko (AI-written, human-approved)

* fix: also clear deliveryContext to prevent lastThreadId repopulation

normalizeSessionEntryDelivery (called on store writes) repopulates
lastThreadId from deliveryContext.threadId. Clearing only the last*
fields is insufficient — deliveryContext must also be cleared when
creating a new isolated session.

✍️ Author: Claude Code with @carrotRakko (AI-written, human-approved)

src/cron/isolated-agent/session.ts

@@ -65,6 +65,19 @@ export function resolveCronSession(params: {
     sessionId,
     updatedAt: params.nowMs,
     systemSent,
+    // When starting a fresh session (forceNew / isolated), clear delivery routing
+    // state inherited from prior sessions. Without this, lastThreadId leaks into
+    // the new session and causes announce-mode cron deliveries to post as thread
+    // replies instead of channel top-level messages.
+    // deliveryContext must also be cleared because normalizeSessionEntryDelivery
+    // repopulates lastThreadId from deliveryContext.threadId on store writes.
+    ...(isNewSession && {
+      lastChannel: undefined,
+      lastTo: undefined,
+      lastAccountId: undefined,
+      lastThreadId: undefined,
+      deliveryContext: undefined,
+    }),
   };
   return { storePath, store, sessionEntry, systemSent, isNewSession };
 }