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/__init__.py

@@ -5,21 +5,23 @@ The bulk of the implementation lives in sibling modules:
   - util:                 thin Docker subprocess wrappers
   - bottle_plan:          DockerBottlePlan
   - bottle_cleanup_plan:  DockerBottleCleanupPlan
-  - bottle:               _DockerBottle handle
+  - bottle:               DockerBottle handle
   - platform:             DockerBottlePlatform
 
-This file only re-exports the platform class so
+This file only re-exports the public names so
 `from claude_bottle.bottles.docker import DockerBottlePlatform` keeps
 working.
 """
 
 from __future__ import annotations
 
+from .bottle import DockerBottle
 from .bottle_cleanup_plan import DockerBottleCleanupPlan
 from .bottle_plan import DockerBottlePlan
 from .platform import DockerBottlePlatform
 
 __all__ = [
+    "DockerBottle",
     "DockerBottleCleanupPlan",
     "DockerBottlePlan",
     "DockerBottlePlatform",