The single docker/__init__.py grew to ~555 lines holding the platform,
its plan classes, the bottle handle, and the runsc probe. Split into:
- util.py : Docker subprocess primitives + runsc_available
- bottle_plan.py : DockerBottlePlan (+ its print method)
- bottle_cleanup_plan.py : DockerBottleCleanupPlan
- bottle.py : _DockerBottle handle class
- platform.py : DockerBottlePlatform (the bulk)
docker/__init__.py becomes a thin re-export shim so existing imports
(claude_bottle.bottles.docker.DockerBottlePlatform, etc.) keep working.
