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/exec/src/lib.rs

@@ -1872,6 +1872,7 @@ async fn resolve_resume_thread_id(
                     request_id: RequestId::Integer(0),
                     params: ThreadListParams {
                         excluded_thread_ids: None,
+                        attached_to_thread_id: None,
                         originators: None,
                         cursor,
                         limit: Some(100),
@@ -1959,6 +1960,7 @@ async fn resolve_resume_thread_id(
                 request_id: RequestId::Integer(0),
                 params: ThreadListParams {
                     excluded_thread_ids: None,
+                    attached_to_thread_id: None,
                     originators: None,
                     cursor,
                     limit: Some(100),