openclaw#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 openclaw#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)

(cherry picked from commit e1df1c6)