feat(cli): add commit command to snapshot running bottle state

Adds `./cli.py commit [<slug>]` which runs `docker commit` on the
active agent container and stores the resulting image tag in per-bottle
state. The next `./cli.py resume <slug>` automatically boots from the
committed snapshot instead of rebuilding from the Dockerfile, preserving
all in-container state across restarts and migrations.

- bottle_state: add write_committed_image / read_committed_image helpers
- docker/util: add commit_container wrapper around `docker commit`
- docker/launch: check for a committed image before the Dockerfile build
  step; fall back to normal build if the image is absent from the daemon
- cli/commit: new command with interactive slug picker; errors clearly on
  non-Docker backends
- 50 new unit tests covering all paths

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bot_bottle/backend/docker/util.py

@@ -152,6 +152,21 @@ def build_image(ref: str, context: str, *, dockerfile: str = "") -> None:
 #         )
 
 
+def commit_container(container_name: str, image_tag: str) -> None:
+    """Run `docker commit <container_name> <image_tag>` to snapshot the
+    running container's filesystem state as a local Docker image."""
+    result = subprocess.run(
+        ["docker", "commit", container_name, image_tag],
+        capture_output=True, text=True, check=False,
+    )
+    if result.returncode != 0:
+        die(
+            f"docker commit {container_name!r} → {image_tag!r} failed: "
+            f"{(result.stderr or '').strip() or '<no stderr>'}"
+        )
+    info(f"committed {container_name!r} → {image_tag!r}")
+
+
 def image_id(ref: str) -> str:
     """Return the content-addressed image ID (e.g.
     `sha256:abcd...`) for `ref`. The smolmachines backend keys its