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/app-server-protocol/src/protocol/v2/thread.rs

@@ -623,6 +623,12 @@ pub struct ThreadForkParams {
     pub developer_instructions: Option<String>,
     #[serde(default, skip_serializing_if = "std::ops::Not::not")]
     pub ephemeral: bool,
+    /// Persist the fork as an attached thread of its resolved source, hidden from ordinary
+    /// lists. The source cannot be deleted while attached threads exist. Requires local storage;
+    /// incompatible with ephemeral and deferGoalContinuation. Omitted or false creates an ordinary fork.
+    #[experimental("thread/fork.attachToSource")]
+    #[serde(default, skip_serializing_if = "std::ops::Not::not")]
+    pub attach_to_source: bool,
     /// Optional client-supplied analytics source classification for this forked thread.
     #[ts(optional = nullable)]
     pub thread_source: Option<ThreadSource>,
@@ -1483,6 +1489,12 @@ pub struct ThreadListParams {
     #[experimental("thread/list.ancestorThreadId")]
     #[ts(optional = nullable)]
     pub ancestor_thread_id: Option<String>,
+    /// Return only persistent threads attached to this thread, including empty threads.
+    /// Mutually exclusive with parentThreadId and ancestorThreadId.
+    /// Omitting this filter keeps attached threads hidden from ordinary listings.
+    #[experimental("thread/list.attachedToThreadId")]
+    #[ts(optional = nullable)]
+    pub attached_to_thread_id: Option<String>,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]