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/schema/json/ClientRequest.json

@@ -4158,8 +4158,7 @@
     "ThreadSortKey": {
       "enum": [
         "created_at",
-        "updated_at",
-        "recency_at"
+        "updated_at"
       ],
       "type": "string"
     },