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_teardown.py

@@ -16,7 +16,7 @@ from pathlib import Path
 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.egress import EgressPlan
@@ -86,6 +86,8 @@ class TestTeardownWarning(unittest.TestCase):
         plan = _plan(self._tmp)
         buf = io.StringIO()
 
+        images = BottleImages(agent="bot-bottle-claude:latest", sidecar="bot-bottle-sidecars:latest")
+
         with mock.patch.object(launch_mod.docker_mod, "build_image"), \
              mock.patch.object(
                  launch_mod, "egress_tls_init",
@@ -115,7 +117,7 @@ class TestTeardownWarning(unittest.TestCase):
              ), \
              contextlib.redirect_stderr(buf):
             provision = mock.Mock(return_value=None)
-            with launch_mod.launch(plan, provision=provision):
+            with launch_mod.launch(plan, images, provision=provision):
                 pass
 
         output = buf.getvalue()