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().

tests/unit/test_docker_launch_committed_image.py

@@ -13,7 +13,7 @@ from typing import Any
 from unittest import mock
 
 from bot_bottle.agent_provider import AgentProvisionPlan
-from bot_bottle.backend import BottleSpec
+from bot_bottle.backend import BottleImages, BottleSpec
 from bot_bottle.backend.docker import launch as launch_mod
 from bot_bottle.backend.docker.bottle_plan import DockerBottlePlan
 from bot_bottle.backend.docker.consolidated_launch import LaunchContext
@@ -115,7 +115,8 @@ class TestLaunchCommittedImage(unittest.TestCase):
         with self._patched(
             committed_tag=committed_tag, image_present=image_present, compose=compose,
         ) as built:
-            with launch_mod.launch(plan, provision=mock.Mock(return_value=None)):
+            images = launch_mod.build_or_load_images(plan)
+            with launch_mod.launch(plan, images, provision=mock.Mock(return_value=None)):
                 pass
         return built
 
@@ -130,8 +131,10 @@ class TestLaunchCommittedImage(unittest.TestCase):
             captured.append(p)
             return {"services": {"agent": {}}}
 
-        with self._patched(committed_tag=_COMMITTED_TAG, image_present=True, compose=compose):
-            with launch_mod.launch(_plan(self._tmp), provision=mock.Mock(return_value=None)):
+        with self._patched(committed_tag=_COMMITTED_TAG, image_present=True, compose=compose) as _:
+            plan = _plan(self._tmp)
+            images = launch_mod.build_or_load_images(plan)
+            with launch_mod.launch(plan, images, provision=mock.Mock(return_value=None)):
                 pass
         self.assertEqual(_COMMITTED_TAG, captured[0].image)