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_data.rs

@@ -152,9 +152,6 @@ pub struct Thread {
     /// Unix timestamp (in seconds) when the thread was last updated.
     #[ts(type = "number")]
     pub updated_at: i64,
-    /// Unix timestamp (in seconds) used for thread recency ordering.
-    #[ts(type = "number | null")]
-    pub recency_at: Option<i64>,
     /// Current runtime status for the thread.
     pub status: ThreadStatus,
     /// [UNSTABLE] Path to the thread on disk.