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/list_threads.rs

@@ -34,7 +34,6 @@ pub(super) async fn list_threads(
     let sort_key = match params.sort_key {
         ThreadSortKey::CreatedAt => codex_rollout::ThreadSortKey::CreatedAt,
         ThreadSortKey::UpdatedAt => codex_rollout::ThreadSortKey::UpdatedAt,
-        ThreadSortKey::RecencyAt => codex_rollout::ThreadSortKey::RecencyAt,
     };
     let sort_direction = match params.sort_direction {
         SortDirection::Asc => codex_rollout::SortDirection::Asc,