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/smolmachines/freezer.py

@@ -0,0 +1,25 @@
+"""SmolmachinesFreezer — snapshot a smolmachines bottle via smolvm pack."""
+
+from __future__ import annotations
+
+from .. import ActiveAgent, Bottle
+from ..freeze import Freezer
+from .smolvm import pack_create_from_vm
+from ...bottle_state import bottle_state_dir
+from ...log import info
+
+
+class SmolmachinesFreezer(Freezer):
+    """Freezes a smolmachines bottle via `smolvm pack create --from-vm`."""
+
+    backend_name = "smolmachines"
+
+    def _freeze(self, agent: ActiveAgent, bottle: Bottle) -> str:
+        output = bottle_state_dir(agent.slug) / "committed-smolmachine"
+        output.parent.mkdir(parents=True, exist_ok=True)
+        pack_create_from_vm(bottle.name, output)
+        artifact = output.with_name(f"{output.name}.smolmachine")
+        return str(artifact)
+
+    def _export_hint(self, slug: str, image_ref: str) -> None:
+        info(f"to export for migration:      cp {image_ref} {slug}.smolmachine")