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/protocol/src/protocol.rs

@@ -3285,6 +3285,9 @@ pub struct SessionMeta {
     pub id: ThreadId,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub forked_from_id: Option<ThreadId>,
+    /// Marks a persisted fork as attached to `forked_from_id`.
+    #[serde(default, skip_serializing_if = "std::ops::Not::not")]
+    pub attached_thread: bool,
     /// Exclusive ordinal inherited from the logical fork parent, independent of `history_base`.
     /// Revert may replace the physical history base while retaining this fork boundary.
     #[serde(default, skip_serializing_if = "Option::is_none")]
@@ -3359,6 +3362,7 @@ impl Default for SessionMeta {
             session_id: id.into(),
             id,
             forked_from_id: None,
+            attached_thread: false,
             forked_from_ordinal_exclusive: None,
             parent_thread_id: None,
             timestamp: String::new(),