Add experimental attached thread forks (#56351)

## What changed

- Add `attachToSource` to `thread/fork` to persist a fork attached to its source without emitting `thread/started`. Attached forks require local storage and cannot use `ephemeral` or `deferGoalContinuation`.
- Hide attached threads from ordinary listings and add `attachedToThreadId` to `thread/list` for explicit discovery, including empty threads. Keep this relationship separate from spawned-agent parent and ancestor filters.
- Persist attachment metadata in rollouts and SQLite so it survives restarts and database recovery. Reject source deletion until its attached threads are deleted.

## Testing

Add coverage for legacy and paginated history, restart and missing or corrupt database recovery, filtered listings and pagination, resume, conflicting options, and deletion safeguards across store instances.

GitOrigin-RevId: 589f3174fc36f6054e75d65818360695645a5913

codex-rs/thread-store/src/thread_metadata_sync.rs

@@ -485,6 +485,7 @@ mod tests {
     async fn create_metadata_records_originator_without_project() {
         let thread_id = ThreadId::new();
         let sync = ThreadMetadataSync::for_create(&CreateThreadParams {
+            attached_thread: false,
             creator_user_id: None,
             creator_account_id: None,
             session_id: thread_id.into(),