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/state/src/migrations.rs

@@ -39,7 +39,3 @@ pub(crate) fn runtime_goals_migrator() -> Migrator {
 pub(crate) fn runtime_memories_migrator() -> Migrator {
     runtime_migrator(&MEMORIES_MIGRATOR)
 }
-
-#[cfg(test)]
-#[path = "migrations_tests.rs"]
-mod tests;