Add Pydantic models and FastAPI router structure for agent and backend

agent/app/routers/runtime.py

@@ -0,0 +1,13 @@
+from fastapi import APIRouter
+
+from app.models import RuntimeResponse
+from app.services.runtime_probe import build_runtime
+from app.services.systemd_manager import SystemdManager
+
+router = APIRouter(tags=['runtime'])
+manager = SystemdManager()
+
+
+@router.get('/runtime', response_model=RuntimeResponse)
+def runtime() -> RuntimeResponse:
+    return build_runtime(manager)