refactor(image-cache): replace warn_if_stale with StaleImageError; add launch template

- image_cache: StaleImageError exception + check_stale/check_stale_path (raise instead of warn)
- BottleBackend.launch: template method (skip_stale flag) that calls _image_stale_checks then _launch_impl
- Each backend: _image_stale_checks delegates to a stale_checks() function in its launch module; _launch_impl replaces launch override
- macos_container: adds image_created_at to util, cached-image support in _build_images, stale_checks
- cli/start.py: catches StaleImageError, prompts interactively, retries with skip_stale=True; headless mode dies on it

tests/unit/test_smolmachines_launch_image.py

@@ -206,8 +206,6 @@ class TestAgentFromPath(unittest.TestCase):
             ), patch.object(
                 _launch_mod.docker_mod, "image_id",
                 return_value=f"sha256:{digest}fffffffffffffffff",
-            ), patch.object(
-                _launch_mod, "warn_if_stale_path",
             ), patch.object(
                 _launch_mod, "_ensure_smolmachine",
             ) as ensure: