refactor: creating providers placeholders and barrel file

server/providers/codex/index.js

@@ -0,0 +1,8 @@
+/**
+ * Codex provider barrel.
+ * Assembles the ProviderAdapter from adapter + sessions.
+ */
+import { normalizeMessage } from './adapter.js';
+import { fetchHistory } from './sessions.js';
+
+export const codexAdapter = { normalizeMessage, fetchHistory };