feat(skills): unified management architecture with SSOT and React Query

- Introduce SSOT (Single Source of Truth) at ~/.cc-switch/skills/
- Add three-app toggle support (Claude/Codex/Gemini) for each skill
- Refactor frontend to use TanStack Query hooks instead of manual state
- Add UnifiedSkillsPanel for managing installed skills with app toggles
- Add useSkills.ts with declarative data fetching hooks
- Extend skills.ts API with unified install/uninstall/toggle methods
- Support importing unmanaged skills from app directories
- Add v2→v3 database migration for new skills table structure

tests/msw/handlers.ts

@@ -36,6 +36,8 @@ const withJson = async <T>(request: Request): Promise<T> => {
 const success = <T>(payload: T) => HttpResponse.json(payload as any);
 
 export const handlers = [
+  http.post(`${TAURI_ENDPOINT}/get_migration_result`, () => success(false)),
+  http.post(`${TAURI_ENDPOINT}/get_skills_migration_result`, () => success(null)),
   http.post(`${TAURI_ENDPOINT}/get_providers`, async ({ request }) => {
     const { app } = await withJson<{ app: AppId }>(request);
     return success(getProviders(app));