refactor: BottleImages dataclass, prelaunch_checks, build_or_load_images

Addresses review comments 3098, 3099, 3100 on PR #336:

- Add BottleImages(agent, sidecar) dataclass to backend/__init__.py.
  Docker/macOS backends use str image refs; smolmachines uses Path
  artifacts. Replaces the singular `image` variable from the canonical
  pattern in comment 3100.

- Replace _image_stale_checks/skip_stale with public prelaunch_checks().
  CLI now calls backend.prelaunch_checks(plan) before backend.launch(plan);
  if StaleImageError is raised and the operator confirms, launch proceeds
  without re-checking. Removes the while-True/skip_stale retry loop.

- Add abstract _build_or_load_images(plan) -> BottleImages to
  BottleBackend. launch() calls it then passes images to _launch_impl.
  Each backend implements both methods.

- Fix comment 3098 (macos-container): _build_images is removed.
  build_or_load_images() has separate fresh/cached code paths — the
  cached path never calls a build helper.

- Update _start_bundle (smolmachines) to accept sidecar_artifact: Path
  directly. Sidecar artifact resolution moves to _sidecar_from_path(),
  called by build_or_load_images alongside _agent_from_path().

bot_bottle/backend/smolmachines/backend.py

@@ -19,7 +19,7 @@ from ...env import ResolvedEnv
 from ...git_gate import GitGatePlan
 from ...supervise import SupervisePlan
 from ...manifest import Manifest
-from .. import ActiveAgent, BottleBackend, BottleSpec
+from .. import ActiveAgent, BottleBackend, BottleImages, BottleSpec
 from . import cleanup as _cleanup
 from . import enumerate as _enumerate
 from . import launch as _launch
@@ -77,14 +77,17 @@ class SmolmachinesBottleBackend(
             stage_dir=stage_dir,
         )
 
-    def _image_stale_checks(self, plan: SmolmachinesBottlePlan) -> None:
+    def prelaunch_checks(self, plan: SmolmachinesBottlePlan) -> None:
         _launch.stale_checks(plan)
 
+    def _build_or_load_images(self, plan: SmolmachinesBottlePlan) -> BottleImages:
+        return _launch.build_or_load_images(plan)
+
     @contextmanager
     def _launch_impl(
-        self, plan: SmolmachinesBottlePlan
+        self, plan: SmolmachinesBottlePlan, images: BottleImages
     ) -> Generator[SmolmachinesBottle, None, None]:
-        with _launch.launch(plan, provision=self.provision) as bottle:
+        with _launch.launch(plan, images, provision=self.provision) as bottle:
             yield bottle
 
     def supervise_mcp_url(self, plan: SmolmachinesBottlePlan) -> str: