feat(app-server, core): return threads by created_at or updated_at (#9247)

Add support for returning threads by either `created_at` OR `updated_at`
descending. Previously core always returned threads ordered by
`created_at`.

This PR:
- updates core to be able to list threads by `updated_at` OR
`created_at` descending based on what the caller wants
- also update `thread/list` in app-server to expose this (default to
`created_at` if not specified)

All existing codepaths (app-server, TUI) still default to `created_at`,
so no behavior change is expected with this PR.

**Implementation**
To sort by `updated_at` is a bit nontrivial (whereas `created_at` is
easy due to the way we structure the folders and filenames on disk,
which are all based on `created_at`).

The most naive way to do this without introducing a cache file or sqlite
DB (which we have to implement/maintain) is to scan files in reverse
`created_at` order on disk, and look at the file's mtime (last modified
timestamp according to the filesystem) until we reach `MAX_SCAN_FILES`
(currently set to 10,000). Then, we can return the most recent N
threads.

Based on some quick and dirty benchmarking on my machine with ~1000
rollout files, calling `thread/list` with limit 50, the `updated_at`
path is slower as expected due to all the I/O:
- updated-at: average 103.10 ms
- created-at: average 41.10 ms

Those absolute numbers aren't a big deal IMO, but we can certainly
optimize this in a followup if needed by introducing more state stored
on disk.

**Caveat**
There's also a limitation in that any files older than `MAX_SCAN_FILES`
will be excluded, which means if a user continues a REALLY old thread,
it's possible to not be included. In practice that should not be too big
of an issue.

If a user makes...
- 1000 rollouts/day → threads older than 10 days won't show up
- 100 rollouts/day → ~100 days

If this becomes a problem for some reason, even more motivation to
implement an updated_at cache.

codex-rs/app-server/README.md

@@ -138,10 +138,11 @@ To branch from a stored session, call `thread/fork` with the `thread.id`. This c
 
 ### Example: List threads (with pagination & filters)
 
-`thread/list` lets you render a history UI. Pass any combination of:
+`thread/list` lets you render a history UI. Results default to `createdAt` (newest first) descending. Pass any combination of:
 
 - `cursor` — opaque string from a prior response; omit for the first page.
 - `limit` — server defaults to a reasonable page size if unset.
+- `sortKey` — `created_at` (default) or `updated_at`.
 - `modelProviders` — restrict results to specific providers; unset, null, or an empty array will include all providers.
 
 Example:
@@ -150,11 +151,12 @@ Example:
 { "method": "thread/list", "id": 20, "params": {
     "cursor": null,
     "limit": 25,
+    "sortKey": "created_at"
 } }
 { "id": 20, "result": {
     "data": [
-        { "id": "thr_a", "preview": "Create a TUI", "modelProvider": "openai", "createdAt": 1730831111 },
-        { "id": "thr_b", "preview": "Fix tests", "modelProvider": "openai", "createdAt": 1730750000 }
+        { "id": "thr_a", "preview": "Create a TUI", "modelProvider": "openai", "createdAt": 1730831111, "updatedAt": 1730831111 },
+        { "id": "thr_b", "preview": "Fix tests", "modelProvider": "openai", "createdAt": 1730750000, "updatedAt": 1730750000 }
     ],
     "nextCursor": "opaque-token-or-null"
 } }