feat: add Skill API endpoints

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/routers/__init__.py

@@ -8,3 +8,4 @@ from app.routers.agent import router as agent_router
 from app.routers.todo import router as todo_router
 from app.routers.settings import router as settings_router
 from app.routers.folder import router as folder_router
+from app.routers.skill import router as skill_router