## What changed
- Add `isPinned` to thread responses and allow `thread/metadata/update` to pin or unpin stored threads.
- Add an `isPinned` filter to `thread/list`, including cursor-based pagination and combinations with relationship filters.
- Persist pin state in SQLite without modifying rollout files, default existing threads to unpinned, and preserve pins through reconciliation and archive transitions.
## Testing
- Cover protocol serialization, database migration and indexing, pin updates, filtered pagination, rollout reconciliation, and archive/unarchive behavior.
GitOrigin-RevId: c2eacabd6dae465e2ae6ce8a169e00740960cdb7
