refactor(docker): fold run_docker into backend/docker/util; drop the docker_cmd shim

docker_cmd.py existed as a top-level module solely so the orchestrator's docker
components could share run_docker without dragging the (then-heavy) backend
layer in. Now that backend/__init__ and backend/docker/__init__ are thin,
importing backend.docker.util costs 6 modules instead of 76, so the shim's whole
reason to exist is gone.

Move run_docker into backend/docker/util.py (where the other docker subprocess
primitives live) and delete docker_cmd.py. Backend siblings import it via
`from .util import run_docker`; the two docker-specific orchestrator modules
(docker_broker, rotate_ca) import `from ..backend.docker.util import run_docker`.
No import cycle (backend.docker.util pulls nothing from orchestrator);
orchestrator.__main__ stays lean at 28 modules. run_docker patch targets are
unchanged (tests patch it in the importing module).

Full unit suite green (2243).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/backend/docker/gateway_provision.py

@@ -17,7 +17,7 @@ from __future__ import annotations
 import re
 from typing import Protocol
 
-from ...docker_cmd import run_docker
+from .util import run_docker
 from ...git_gate import GitGatePlan, git_gate_render_provision
 
 # bottle ids index the gateway's per-bottle repo + creds dirs; they land in