## Why
Title and content searches exclude attached threads, preventing clients from finding their saved conversations alongside ordinary threads.
## What changed
- Add `includeAttachedThreads` to `thread/list` and `thread/search`. For `thread/list`, require a non-empty `searchTerm` and preserve the usual provider and source filters.
- Expose `attachedToThreadId` on thread responses to identify the owning parent independently of fork and subagent lineage.
- Support inclusive searches through SQLite and filesystem fallback, repairing missing database entries with saved thread names during title searches.
## Testing
Add regression coverage for opt-in title and content searches, attached-parent metadata after restart, default search filters, filesystem searches without SQLite, and discovery and repair of unindexed title matches.
GitOrigin-RevId: ac12ec3c730ad7f1722bdf8e21600d17f12017f0