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/app-server-protocol/src/protocol/v2/thread.rs

@@ -1086,7 +1086,6 @@ pub enum ThreadSourceKind {
 pub enum ThreadSortKey {
     CreatedAt,
     UpdatedAt,
-    RecencyAt,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, Copy, PartialEq, JsonSchema, TS)]