refactor(docker): split bottles/docker/__init__.py into sibling modules

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.

claude_bottle/bottles/docker/bottle_cleanup_plan.py

@@ -0,0 +1,36 @@
+"""DockerBottleCleanupPlan — concrete subclass of BottleCleanupPlan.
+
+Holds the tuples of container and network names that
+DockerBottlePlatform.cleanup will remove. The y/N preflight reads
+these via `print`; the CLI short-circuits via `empty`.
+"""
+
+from __future__ import annotations
+
+import sys
+from dataclasses import dataclass
+
+from ...log import info
+from .. import BottleCleanupPlan
+
+
+@dataclass(frozen=True)
+class DockerBottleCleanupPlan(BottleCleanupPlan):
+    """Resources DockerBottlePlatform.cleanup will remove. Produced by
+    `prepare_cleanup` from a snapshot of `docker ps -a` + `docker
+    network ls`; sorted so the y/N output is stable."""
+
+    containers: tuple[str, ...]
+    networks: tuple[str, ...]
+
+    @property
+    def empty(self) -> bool:
+        return not self.containers and not self.networks
+
+    def print(self) -> None:
+        print(file=sys.stderr)
+        for name in self.containers:
+            info(f"container: {name}")
+        for name in self.networks:
+            info(f"network:   {name}")
+        print(file=sys.stderr)