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/core/src/codex_delegate.rs

@@ -103,6 +103,7 @@ pub(crate) async fn run_codex_thread_interactive(
     let mut thread_extension_init = codex_extension_api::ExtensionDataInit::default();
     thread_extension_init.insert(isolation);
     let spawn_result = Session::spawn(SessionSpawnArgs {
+        attached_thread: false,
         startup: Some(Arc::clone(&startup)),
         config,
         prompt_cache_key: parent_session