Add Pydantic models and FastAPI router structure for agent and backend

agent/app/routers/health.py

@@ -0,0 +1,21 @@
+from fastapi import APIRouter
+
+from app.models import HealthResponse
+from app.services.runtime_probe import build_health
+
+router = APIRouter(tags=['health'])
+
+
+@router.get('/live', response_model=HealthResponse)
+def live() -> HealthResponse:
+    return build_health()
+
+
+@router.get('/ready', response_model=HealthResponse)
+def ready() -> HealthResponse:
+    return build_health()
+
+
+@router.get('/health', response_model=HealthResponse)
+def health() -> HealthResponse:
+    return build_health()