## Why
Permanent thread deletion needs to remove persisted agent message board data even when the message board feature has been disabled.
## What changed
- Add a cleanup hook for single and batch thread deletion, invoked after reference and writer checks but before rollout removal so cleanup failures remain retryable.
- Remove the deleted roots' channels, posts, and subscriptions regardless of the current feature setting.
- Record board tombstones and check them within write transactions to prevent delayed writes from recreating deleted data.
## Testing
Add an app-server regression test that creates a board, archives its root thread, restarts with the feature disabled, and verifies that permanent deletion removes the persisted channels, posts, and subscriptions.
GitOrigin-RevId: 4ea71559a8df0814dffda23a056a9bbaf6e0a0cb
jif
73129716c2