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/analytics/src/client_tests.rs

@@ -285,7 +285,6 @@ fn sample_thread(thread_id: &str) -> Thread {
         model_provider: "openai".to_string(),
         created_at: 1,
         updated_at: 2,
-        recency_at: Some(2),
         status: AppServerThreadStatus::Idle,
         path: None,
         cwd: test_path_buf("/tmp").abs(),