Refac: Centralize storage file management (#4078)

Co-authored-by: Taylor Mullen <ntaylormullen@google.com>

packages/cli/src/config/extension.test.ts

@@ -10,7 +10,6 @@ import * as os from 'os';
 import * as path from 'path';
 import {
   EXTENSIONS_CONFIG_FILENAME,
-  EXTENSIONS_DIRECTORY_NAME,
   annotateActiveExtensions,
   loadExtensions,
 } from './extension.js';
@@ -23,6 +22,8 @@ vi.mock('os', async (importOriginal) => {
   };
 });
 
+const EXTENSIONS_DIRECTORY_NAME = path.join('.gemini', 'extensions');
+
 describe('loadExtensions', () => {
   let tempWorkspaceDir: string;
   let tempHomeDir: string;