app-server: Emit thread archive/unarchive notifications (#12030)

* Add v2 server notifications `thread/archived` and `thread/unarchived`
with a `threadId` payload.
* Wire new events into `thread/archive` and `thread/unarchive` success
paths.
* Update app-server protocol/schema/docs accordingly.

Testing:
- Updated archive/unarchive end-to-end tests to verify both
notifications are emitted with the expected thread id payload.

codex-rs/app-server-protocol/schema/json/v2/SkillsRemoteWriteResponse.json

@@ -14,4 +14,4 @@
   ],
   "title": "SkillsRemoteWriteResponse",
   "type": "object"
-}
+}