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_macos_container_launch.py

@@ -81,7 +81,7 @@ def _plan(
         provisioned_env={"CODEX_HOME": "/run/codex-home"},
     )
     return cast(MacosContainerBottlePlan, SimpleNamespace(
-        spec=SimpleNamespace(),
+        spec=SimpleNamespace(image_policy="fresh"),
         manifest=_MANIFEST,
         stage_dir=stage_dir,
         slug="dev-abc",
@@ -292,7 +292,7 @@ class TestMacosContainerLaunchArgv(unittest.TestCase):
 
 def _build_plan(stage_dir: Path) -> MacosContainerBottlePlan:
     return MacosContainerBottlePlan(
-        spec=cast(BottleSpec, SimpleNamespace()),
+        spec=cast(BottleSpec, SimpleNamespace(image_policy="fresh")),
         manifest=_MANIFEST,
         stage_dir=stage_dir,
         git_gate_plan=cast(GitGatePlan, SimpleNamespace(upstreams=())),