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/exec/src/lib_tests.rs

@@ -342,7 +342,6 @@ fn turn_items_for_thread_returns_matching_turn_items() {
         model_provider: "openai".to_string(),
         created_at: 0,
         updated_at: 0,
-        recency_at: Some(0),
         status: codex_app_server_protocol::ThreadStatus::Idle,
         path: None,
         cwd: test_path_buf("/tmp/project").abs(),
@@ -761,7 +760,6 @@ fn sample_thread_start_response() -> ThreadStartResponse {
             model_provider: "openai".to_string(),
             created_at: 0,
             updated_at: 0,
-            recency_at: Some(0),
             status: codex_app_server_protocol::ThreadStatus::Idle,
             path: Some(PathBuf::from("/tmp/rollout.jsonl")),
             cwd: test_path_buf("/tmp").abs(),