Update documentation after refactoring: AGENTS.md, GAME_BRIEFING.md, skills.md

- Updated AGENTS.md to include store/ directory and clarify store architecture
- Updated GAME_BRIEFING.md Code Architecture section with store/ and legacy store info
- Updated skills.md with skill state management information
- Includes other refactoring changes (store hooks, component updates, etc.)

vitest.config.ts

@@ -2,7 +2,7 @@ import { defineConfig } from 'vitest/config';
 
 export default defineConfig({
   test: {
-    environment: 'node',
+    environment: 'jsdom',
     globals: true,
     setupFiles: ['./src/test/setup.ts'],
     include: ['src/**/*.{test,spec}.ts'],