fix: shared runtime helper must live where every image packages it

services/_runtime.py crashed the worker and reconciler on boot with
`ModuleNotFoundError: No module named 'services._runtime'`, while every unit and
integration gate was green and the API rolled out fine.

The cause is packaging, not code. Each service Dockerfile copies only its own
`services/<svc>/` subdir — `services/` itself is a namespace package with no
__init__.py, so a file added at the `services/` root is never copied into any
image. Tests import from the source tree, where the file exists, so nothing
below the image boundary could catch it. The API survived only because it does
not import the helper.

Moved to svcforge_core.runtime, which `COPY libs/ libs/` packages into every
image, next to adapters/tempyaml.py for the same reason.

Added an import smoke test to the image job: `docker run --entrypoint python
<image> -c "import services.<svc>.main"` loads the whole transitive graph inside
the built image and fails the build before the digest is pushed. This is the
one check the test suite structurally cannot perform — it runs against source,
the image is a different filesystem — and it is exactly the gap this bug fell
through.

services/worker/main.py

@@ -15,7 +15,6 @@ from collections.abc import Awaitable
 
 from opentelemetry import trace
 
-from services._runtime import install_stop_signals, sleep_or_stop
 from services.worker.deps import WorkerDeps
 from services.worker.handlers import HANDLERS
 from svcforge_core import obs
@@ -27,6 +26,7 @@ from svcforge_core.domain.models import Task, TaskKind
 from svcforge_core.repo.db import make_pool
 from svcforge_core.repo.instances import InstanceRepo
 from svcforge_core.repo.tasks import TaskRepo
+from svcforge_core.runtime import install_stop_signals, sleep_or_stop
 from svcforge_core.settings import Settings, load_settings
 
 log = obs.get_logger("svcforge.worker")