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_smolmachines_provision.py

@@ -546,24 +546,22 @@ class TestLaunchResourceWiring(unittest.TestCase):
             ),
         )
 
+        sidecar_artifact = Path("/cache/sidecar.smolmachine")
+
         with patch(
             "bot_bottle.backend.smolmachines.launch._provision_git_gate_keys",
             return_value=plan,
         ), patch(
-            "bot_bottle.backend.smolmachines.launch._ensure_smolmachine",
-            return_value=Path("/cache/sidecar.smolmachine"),
-        ) as ensure, patch(
             "bot_bottle.backend.smolmachines.launch._bundle.start_bundle_vm",
             return_value=raw_launch,
         ) as start_vm, patch(
             "bot_bottle.backend.smolmachines.launch._forward.start_forwarder",
             return_value=handle,
         ) as start_forwarder:
-            stamped = _launch._start_bundle(plan, "net", "127.0.0.16", stack)
+            stamped = _launch._start_bundle(plan, "net", "127.0.0.16", sidecar_artifact, stack)
 
-        ensure.assert_called_once()
         start_vm.assert_called_once()
-        self.assertEqual(Path("/cache/sidecar.smolmachine"), start_vm.call_args.kwargs["from_path"])
+        self.assertEqual(sidecar_artifact, start_vm.call_args.kwargs["from_path"])
         specs = start_forwarder.call_args.args[0]
         self.assertEqual(
             (