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/launch.py

@@ -49,6 +49,7 @@ from ...bottle_state import (
     git_gate_state_dir,
     read_committed_image,
 )
+from .. import BottleImages
 from . import loopback_alias as _loopback
 from . import port_forward as _forward
 from . import sidecar_bundle as _bundle
@@ -76,13 +77,32 @@ _GIT_HTTP_PORT = 9420
 _SUPERVISE_PORT = SUPERVISE_PORT
 
 
+def build_or_load_images(plan: SmolmachinesBottlePlan) -> BottleImages:
+    """Return pre-built or freshly built smolmachine artifact paths."""
+    return BottleImages(
+        agent=_agent_from_path(plan),
+        sidecar=_sidecar_from_path(plan),
+    )
+
+
+def _sidecar_from_path(plan: SmolmachinesBottlePlan) -> Path:
+    """Return the sidecar bundle artifact path, building it if needed."""
+    if _image_policy(plan) == "cached":
+        return _cached_smolmachine(_bundle.SIDECAR_BUNDLE_IMAGE, label="sidecar")
+    return _ensure_smolmachine(
+        _bundle.SIDECAR_BUNDLE_IMAGE,
+        dockerfile=_bundle.SIDECAR_BUNDLE_DOCKERFILE,
+    )
+
+
 @contextmanager
 def launch(
     plan: SmolmachinesBottlePlan,
+    images: BottleImages,
     *,
     provision: Callable[[SmolmachinesBottlePlan, "SmolmachinesBottle"], str | None],
 ) -> Generator[SmolmachinesBottle, None, None]:
-    """Build + run the bottle and yield a handle; tear everything
+    """Run the bottle from pre-built images and yield a handle; tear everything
     down on exit. Errors during bringup unwind any partial state
     via the ExitStack."""
     stack = ExitStack()
@@ -90,11 +110,9 @@ def launch(
         loopback_ip, network = _allocate_resources(plan, stack)
         plan = _mint_certs(plan)
         proxy_host = loopback_ip
-        plan = _start_bundle(plan, network, proxy_host, stack)
+        plan = _start_bundle(plan, network, proxy_host, Path(images.sidecar), stack)
 
-        agent_from_path = _agent_from_path(plan)
-
-        _launch_vm(plan, agent_from_path, proxy_host, stack)
+        _launch_vm(plan, Path(images.agent), proxy_host, stack)
         _init_vm(plan)
 
         bottle = SmolmachinesBottle(
@@ -171,24 +189,18 @@ def _start_bundle(
     plan: SmolmachinesBottlePlan,
     network: str,
     proxy_host: str,
+    sidecar_artifact: Path,
     stack: ExitStack,
 ) -> SmolmachinesBottlePlan:
-    """Build the BundleLaunchSpec, start the sidecar VM, wrap its raw
-    smolVM-published loopback ports with per-bottle forwarders, stamp
-    agent URLs from those forwarder ports, and register teardown."""
+    """Build the BundleLaunchSpec, start the sidecar VM from the pre-resolved
+    artifact, wrap its raw smolVM-published loopback ports with per-bottle
+    forwarders, stamp agent URLs from those forwarder ports, and register teardown."""
     plan = _provision_git_gate_keys(plan)
     bundle_spec = _bundle_launch_spec(plan, network, proxy_host)
     token_env = _resolve_token_env(plan, dict(os.environ))
-    if _image_policy(plan) == "cached":
-        artifact = _cached_smolmachine(bundle_spec.image, label="sidecar")
-    else:
-        artifact = _ensure_smolmachine(
-            bundle_spec.image,
-            dockerfile=_bundle.SIDECAR_BUNDLE_DOCKERFILE,
-        )
     launch = _bundle.start_bundle_vm(
         bundle_spec,
-        from_path=artifact,
+        from_path=sidecar_artifact,
         host_env={**os.environ, **token_env},
     )
     stack.callback(_bundle.stop_bundle_vm, plan.slug)