Revert thread recencyAt for sidebar ordering (#28655)

## Why

Revert #27910 to remove the newly introduced thread `recencyAt`
persistence and API behavior from `main`.

## What changed

This reverts commit `fac3158c2a783095768076489815f361fa9b0db4`,
including the state migration, thread-store propagation, app-server API
surface, generated schemas, and related tests.

## Validation

Not run before opening; relying on CI for the initial fast signal.

codex-rs/thread-store/src/local/unarchive_thread.rs

@@ -196,6 +196,5 @@ mod tests {
             .expect("thread metadata should exist");
         assert_eq!(updated.rollout_path, restored_path);
         assert_eq!(updated.archived_at, None);
-        assert_eq!(updated.recency_at, metadata.recency_at);
     }
 }