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/in_memory.rs

@@ -535,9 +535,6 @@ fn stored_thread_from_state(
         updated_at: metadata
             .and_then(|metadata| metadata.updated_at)
             .unwrap_or_else(Utc::now),
-        recency_at: metadata
-            .and_then(|metadata| metadata.advance_recency_at.or(metadata.updated_at))
-            .unwrap_or_else(Utc::now),
         archived_at: None,
         cwd: metadata
             .and_then(|metadata| metadata.cwd.clone())