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/local/update_thread_metadata.rs

@@ -204,7 +204,6 @@ async fn apply_metadata_update(
                     .map_err(|err| ThreadStoreError::Internal {
                         message: format!("failed to read thread metadata for {thread_id}: {err}"),
                     })?;
-            let advance_recency_at = patch.advance_recency_at;
             if existing.is_none() && rollout_path.is_none() {
                 let resolved = resolve_rollout_path(store, thread_id, include_archived).await?;
                 rollout_path_archived = resolved.archived;
@@ -261,11 +260,6 @@ async fn apply_metadata_update(
             if let Some(updated_at) = patch.updated_at {
                 metadata.updated_at = updated_at;
             }
-            if existing.is_none()
-                && let Some(recency_at) = advance_recency_at
-            {
-                metadata.recency_at = recency_at;
-            }
             if let Some(source) = patch.source {
                 metadata.source = enum_to_string(&source);
             }
@@ -316,18 +310,6 @@ async fn apply_metadata_update(
                 .map_err(|err| ThreadStoreError::Internal {
                     message: format!("failed to update thread metadata for {thread_id}: {err}"),
                 })?;
-            if existing.is_some()
-                && let Some(recency_at) = advance_recency_at
-            {
-                state_db
-                    .touch_thread_recency_at(thread_id, recency_at)
-                    .await
-                    .map_err(|err| ThreadStoreError::Internal {
-                        message: format!(
-                            "failed to advance thread recency_at for {thread_id}: {err}"
-                        ),
-                    })?;
-            }
             if let Some(memory_mode) = patch.memory_mode {
                 state_db
                     .set_thread_memory_mode(thread_id, memory_mode_as_str(memory_mode))