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/common.rs

@@ -2501,7 +2501,6 @@ mod tests {
                     model_provider: "openai".to_string(),
                     created_at: 1,
                     updated_at: 2,
-                    recency_at: Some(3),
                     status: v2::ThreadStatus::Idle,
                     path: None,
                     cwd: cwd.clone(),
@@ -2545,7 +2544,6 @@ mod tests {
                         "modelProvider": "openai",
                         "createdAt": 1,
                         "updatedAt": 2,
-                        "recencyAt": 3,
                         "status": {
                             "type": "idle"
                         },