- claude_bottle/bottles/docker.py -> claude_bottle/bottles/docker/__init__.py
- claude_bottle/docker.py -> claude_bottle/bottles/docker/util.py
The host-side Docker primitives (require_docker, slugify, image_exists,
container_exists, build_image, build_image_with_cwd) lived at the top
of the package as if cross-cutting, but they are Docker-specific.
Moving them into bottles/docker/util.py colocates them with the
platform that uses them and clears the top-level namespace.
Imports in bottles/docker/__init__.py shift to `from . import util as
docker_mod` so the existing call sites (`docker_mod.require_docker()`,
etc.) keep working unchanged.
