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

bot_bottle/backend/smolmachines/backend.py

@@ -77,8 +77,11 @@ class SmolmachinesBottleBackend(
             stage_dir=stage_dir,
         )
 
+    def _image_stale_checks(self, plan: SmolmachinesBottlePlan) -> None:
+        _launch.stale_checks(plan)
+
     @contextmanager
-    def launch(
+    def _launch_impl(
         self, plan: SmolmachinesBottlePlan
     ) -> Generator[SmolmachinesBottle, None, None]:
         with _launch.launch(plan, provision=self.provision) as bottle: