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_macos_container_launch.py

@@ -11,7 +11,7 @@ from typing import cast
 from unittest.mock import patch
 
 from bot_bottle.agent_provider import AgentProvisionPlan
-from bot_bottle.backend import BottleSpec
+from bot_bottle.backend import BottleImages, BottleSpec
 from bot_bottle.backend.macos_container import launch
 from bot_bottle.backend.macos_container.bottle_plan import MacosContainerBottlePlan
 from bot_bottle.egress import EgressPlan
@@ -339,9 +339,9 @@ class TestMacosContainerLaunchCommittedImage(unittest.TestCase):
         ), patch.object(
             launch.container_mod, "build_image", side_effect=fake_build,
         ), patch.object(launch, "info"):
-            updated = launch._build_images(plan)
+            images = launch.build_or_load_images(plan)
 
-        self.assertEqual("bot-bottle-committed-dev-abc:latest", updated.image)
+        self.assertEqual("bot-bottle-committed-dev-abc:latest", images.agent)
         self.assertEqual(1, len(calls))
         self.assertEqual(launch.SIDECAR_BUNDLE_IMAGE, calls[0][0])
 
@@ -360,9 +360,9 @@ class TestMacosContainerLaunchCommittedImage(unittest.TestCase):
         ), patch.object(
             launch.container_mod, "build_image", side_effect=fake_build,
         ):
-            updated = launch._build_images(plan)
+            images = launch.build_or_load_images(plan)
 
-        self.assertEqual("bot-bottle-agent:latest", updated.image)
+        self.assertEqual("bot-bottle-agent:latest", images.agent)
         self.assertEqual(2, len(calls))
         self.assertEqual("bot-bottle-agent:latest", calls[1][0])
 
@@ -395,7 +395,7 @@ class TestMacosContainerLaunchCachedImages(unittest.TestCase):
             launch, "die", side_effect=Die(),
         ):
             with self.assertRaises(Die):
-                launch._build_images(plan)
+                launch.build_or_load_images(plan)
 
     def test_cached_mode_dies_when_sidecar_image_missing(self) -> None:
         plan = self._cached_plan()
@@ -411,7 +411,7 @@ class TestMacosContainerLaunchCachedImages(unittest.TestCase):
             launch, "die", side_effect=Die(),
         ):
             with self.assertRaises(Die):
-                launch._build_images(plan)
+                launch.build_or_load_images(plan)
 
     def test_cached_mode_both_present_returns_unchanged_plan(self) -> None:
         plan = self._cached_plan()
@@ -422,10 +422,10 @@ class TestMacosContainerLaunchCachedImages(unittest.TestCase):
         ), patch.object(
             launch.container_mod, "build_image",
         ) as build, patch.object(launch, "info"):
-            result = launch._build_images(plan)
+            images = launch.build_or_load_images(plan)
 
         build.assert_not_called()
-        self.assertEqual(plan.image, result.image)
+        self.assertEqual(plan.image, images.agent)
 
     def test_committed_image_plus_cached_skips_sidecar_build(self) -> None:
         plan = self._cached_plan()
@@ -437,11 +437,11 @@ class TestMacosContainerLaunchCachedImages(unittest.TestCase):
         ), patch.object(
             launch.container_mod, "build_image",
         ) as build, patch.object(launch, "info"):
-            result = launch._build_images(plan)
+            images = launch.build_or_load_images(plan)
 
         # In cached mode with a committed image, no builds should fire.
         build.assert_not_called()
-        self.assertEqual("bot-bottle-committed-dev-abc:latest", result.image)
+        self.assertEqual("bot-bottle-committed-dev-abc:latest", images.agent)
 
 
 if __name__ == "__main__":