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.py

@@ -0,0 +1,44 @@
+"""_DockerBottle — concrete Bottle handle yielded by
+DockerBottlePlatform.launch.
+
+Holds the container name plus the in-container prompt path so
+exec_claude can transparently add --append-system-prompt-file when a
+prompt was provisioned.
+"""
+
+from __future__ import annotations
+
+import subprocess
+
+
+class _DockerBottle:
+    """Concrete Bottle for Docker."""
+
+    def __init__(self, container: str, teardown, prompt_path_in_container: str | None):
+        self.name = container
+        self._teardown = teardown
+        self._prompt_path = prompt_path_in_container
+        self._closed = False
+
+    def exec_claude(self, argv: list[str], *, tty: bool = True) -> int:
+        full_argv = list(argv)
+        if self._prompt_path:
+            full_argv.extend(["--append-system-prompt-file", self._prompt_path])
+        cmd = ["docker", "exec"]
+        if tty:
+            cmd.append("-it")
+        cmd.extend([self.name, "claude", *full_argv])
+        return subprocess.run(cmd).returncode
+
+    def cp_in(self, host_path: str, container_path: str) -> None:
+        subprocess.run(
+            ["docker", "cp", host_path, f"{self.name}:{container_path}"],
+            stdout=subprocess.DEVNULL,
+            check=True,
+        )
+
+    def close(self) -> None:
+        if self._closed:
+            return
+        self._closed = True
+        self._teardown()