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/app-server/src/thread_status.rs

@@ -897,7 +897,6 @@ mod tests {
             model_provider: "mock-provider".to_string(),
             created_at: 0,
             updated_at: 0,
-            recency_at: Some(0),
             status: ThreadStatus::NotLoaded,
             path: None,
             cwd: test_path_buf("/tmp").abs(),