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/v2/ThreadMetadataUpdateResponse.json

@@ -870,14 +870,6 @@
           "description": "Usually the first user message in the thread, if available.",
           "type": "string"
         },
-        "recencyAt": {
-          "description": "Unix timestamp (in seconds) used for thread recency ordering.",
-          "format": "int64",
-          "type": [
-            "integer",
-            "null"
-          ]
-        },
         "sessionId": {
           "description": "Session id shared by threads that belong to the same session tree.",
           "type": "string"