refactor(bottles): Bottle becomes an ABC; DockerBottle inherits

Bottle was the only Protocol in an otherwise-ABC family
(BottlePlan, BottleCleanupPlan, BottlePlatform are all ABCs).
Convert to an ABC with abstract exec_claude / cp_in / close,
matching the rest of the hierarchy.

Rename _DockerBottle -> DockerBottle: the underscore was a
default-Python-private instinct that doesn't match the sibling
plan classes (DockerBottlePlan, DockerBottleCleanupPlan), all of
which are equally "only constructed by the platform" and yet
public-by-name.

Re-export DockerBottle from claude_bottle.bottles.docker.

claude_bottle/bottles/docker/bottle.py

@@ -1,4 +1,4 @@
-"""_DockerBottle — concrete Bottle handle yielded by
+"""DockerBottle — concrete Bottle handle yielded by
 DockerBottlePlatform.launch.
 
 Holds the container name plus the in-container prompt path so
@@ -10,8 +10,10 @@ from __future__ import annotations
 
 import subprocess
 
+from .. import Bottle
 
-class _DockerBottle:
+
+class DockerBottle(Bottle):
     """Concrete Bottle for Docker."""
 
     def __init__(self, container: str, teardown, prompt_path_in_container: str | None):