refactor(commit): introduce Freezer class hierarchy across backends

Adds a Freezer ABC (backend/freeze.py) that encapsulates the
stop-commit-mark-preserved flow for all backends, following the same
pattern as BottleBackend. Each backend gets its own Freezer subclass:

  DockerFreezer           — docker commit
  MacosContainerFreezer   — container export + image rebuild; prompts
                            to stop if the container is running
  SmolmachinesFreezer     — smolvm pack create --from-vm

The base class owns write_committed_image, mark_preserved, and the
resume hint. Subclasses implement _freeze() and optionally override
_export_hint() for migration instructions.

Freezer.commit(agent, bottle) is the primary entry point for use
within a live launch context. Freezer.commit_slug(slug) is a
convenience wrapper for cmd_commit, which no longer branches on
backend names itself.

get_freezer(backend_name) is the factory, analogous to
get_bottle_backend(). CommitCancelled is raised by MacosContainerFreezer
when the user declines the stop prompt; cmd_commit catches it and
returns 0.

bot_bottle/backend/__init__.py

@@ -526,6 +526,11 @@ from .docker import DockerBottleBackend  # noqa: E402  # pylint: disable=wrong-i
 from .macos_container import MacosContainerBottleBackend  # noqa: E402  # pylint: disable=wrong-import-position
 from .smolmachines import SmolmachinesBottleBackend  # noqa: E402  # pylint: disable=wrong-import-position
 
+# Freezer is imported after the backend classes for the same reason —
+# Freezer.commit_slug constructs ActiveAgent, so the dataclass must be
+# fully defined first.
+from .freeze import CommitCancelled, Freezer, get_freezer  # noqa: E402  # pylint: disable=wrong-import-position
+
 
 # The dict is heterogeneous: each value is a BottleBackend specialized
 # over its own plan type. Concrete plan types are erased here because
@@ -613,9 +618,12 @@ __all__ = [
     "BottleCleanupPlan",
     "BottlePlan",
     "BottleSpec",
+    "CommitCancelled",
     "ExecResult",
+    "Freezer",
     "enumerate_active_agents",
     "get_bottle_backend",
+    "get_freezer",
     "has_backend",
     "known_backend_names",
 ]