Add field to Thread object for the latest rename set for a given thread (#12301)

Exposes through the app server updated names set for a thread. This
enables other surfaces to use the core as the source of truth for thread
naming. `threadName` is gathered using the helper functions used to
interact with `session_index.jsonl`, and is hydrated in:
- `thread/list`
- `thread/read`
- `thread/resume`
- `thread/unarchive`
- `thread/rollback`

We don't do this for `thread/start` and `thread/fork`.

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

@@ -2277,6 +2277,8 @@ pub struct Thread {
     pub agent_role: Option<String>,
     /// Optional Git metadata captured when the thread was created.
     pub git_info: Option<GitInfo>,
+    /// Optional user-facing thread title.
+    pub name: Option<String>,
     /// Only populated on `thread/resume`, `thread/rollback`, `thread/fork`, and `thread/read`
     /// (when `includeTurns` is true) responses.
     /// For all other responses and notifications returning a Thread,