## Why
Concurrent parent compaction can remove evidence between Guardian checkpoint selection and prompt construction. New user instructions or answers received during a review can also invalidate an approval based on earlier authorization.
## What changed
- With `GuardianThreadContext` enabled, use the same parent history snapshot for checkpoint selection and prompt evidence.
- Cancel completed allow decisions if the owning session's user-message revision or root authorization version changed during the review, even without cacheable review evidence.
## Testing
Add regression coverage for evidence preservation during concurrent compaction and aborted approvals after new user instructions or verified answers. Extend reviewer context rollover coverage to exercise `GuardianThreadContext`.
GitOrigin-RevId: 1c2c82197a27c0d3d45b6fe08d200a835d80cc30
felixxia-oai
5b85aea979