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/state/src/extract.rs

@@ -544,7 +544,6 @@ mod tests {
             rollout_path: PathBuf::from("/tmp/a.jsonl"),
             created_at,
             updated_at: created_at,
-            recency_at: created_at,
             source: "cli".to_string(),
             thread_source: None,
             agent_path: None,