refactor(bottles): make docker a package; absorb top-level docker.py

- 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.

claude_bottle/bottles/docker/__init__.py

@@ -27,18 +27,18 @@ from dataclasses import dataclass
 from pathlib import Path
 from typing import Iterator
 
-from .. import docker as docker_mod
+from ... import network as network_mod
-from .. import network as network_mod
+from ... import pipelock
-from .. import pipelock
+from ... import skills as skills_mod
-from .. import skills as skills_mod
+from ... import ssh as ssh_mod
-from .. import ssh as ssh_mod
+from ...env_resolve import env_resolve
-from ..env_resolve import env_resolve
+from ...log import die, info
-from ..log import die, info
+from .. import BottleCleanupPlan, BottlePlan, BottlePlatform, BottleSpec
-from . import BottleCleanupPlan, BottlePlan, BottlePlatform, BottleSpec
+from . import util as docker_mod
 
 
 # Where the repo root lives, for `docker build` context. Computed once.
-_REPO_DIR = str(Path(__file__).resolve().parent.parent.parent)
+_REPO_DIR = str(Path(__file__).resolve().parent.parent.parent.parent)
 
 
 # --- Runtime detection -----------------------------------------------------

claude_bottle/bottles/docker/util.py

@@ -1,4 +1,6 @@
-"""Docker helpers. Build/inspect primitives shared by the CLI."""
+"""Docker host-side primitives used by DockerBottlePlatform: probing
+for docker on PATH, slugifying agent names, checking image/container
+existence, and building images."""
 
 from __future__ import annotations
 
@@ -7,7 +9,7 @@ import shutil
 import subprocess
 from typing import Iterable
 
-from .log import die, info
+from ...log import die, info
 
 
 def require_docker() -> None: