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/schema/json/codex_app_server_protocol.schemas.json

@@ -15553,6 +15553,13 @@
             "description": "Model provider used for this thread (for example, 'openai').",
             "type": "string"
           },
+          "name": {
+            "description": "Optional user-facing thread title.",
+            "type": [
+              "string",
+              "null"
+            ]
+          },
           "path": {
             "description": "[UNSTABLE] Path to the thread on disk.",
             "type": [