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_docker_launch_committed_image.py

@@ -105,10 +105,6 @@ class TestLaunchCommittedImage(unittest.TestCase):
             launch_mod.docker_mod, "image_exists", return_value=image_present,
         ), mock.patch.object(
             launch_mod.docker_mod, "build_image", side_effect=fake_build,
-        ), mock.patch.object(
-            launch_mod.docker_mod, "image_created_at",
-        ), mock.patch.object(
-            launch_mod, "warn_if_stale",
         ), mock.patch.object(
             launch_mod, "egress_tls_init",
             return_value=(Path("/egress_ca"), Path("/egress_cert")),