updated AGENTS.md to match backend state

AGENTS.md

@@ -134,9 +134,12 @@ for it exists:
 - **The grid is not wired up.** [src/components/BrowseSaveDataGrid.tsx](src/components/BrowseSaveDataGrid.tsx)
   renders two hardcoded rows and never calls the backend. `/browse?id=…` only
   uses the id for the download button so far.
-- **The backend does not speak the new upload format yet.** It has to accept the
-  zstd compressed tar described above and serve `getSaveFile`; until it does,
-  uploads fail against the running service.
+- **The backend only supports EU4.** It accepts the zstd compressed tar
+  described above, rejects anything else, and serves `getSaveFile`. Uploads
+  for other games, Stellaris included, are rejected with 400 until the backend
+  gets a parser for them. It also enforces its own upload limits and answers
+  413 when a save is over them. Those limits are separate from the ones in
+  `runtimeConfig.json`, so keep the two in sync.
 - **Auth does not work.** `AuthService` posts to `/api/auth/login` and
   `/api/auth/register`, which do not exist as routes. The service is slated for
   removal in favour of Discord OAuth.