refactor(backend): introduce BottleProvisioner ABC + DockerBottleProvisioner

Lift the file-copying-into-the-running-container step out of
DockerBottleBackend._provision_container into its own class. The
backend now holds a DockerBottleProvisioner instance and delegates
the post-launch provisioning to it.

  - BottleProvisioner (abstract) in backend/__init__.py with a
    `provision(plan, target) -> str | None` method.
  - DockerBottleProvisioner (concrete) in backend/docker/provisioner.py
    inheriting from the base, narrowing plan to DockerBottlePlan via
    isinstance, and carrying the prompt/skills/SSH/.git copy logic
    unchanged.
  - DockerBottleBackend keeps a class-level DockerBottleProvisioner()
    and calls self._provisioner.provision(plan, container) from launch.
    _provision_container method removed.

No behavior change.

claude_bottle/backend/docker/__init__.py

@@ -7,6 +7,7 @@ The bulk of the implementation lives in sibling modules:
   - bottle_plan:          DockerBottlePlan
   - bottle_cleanup_plan:  DockerBottleCleanupPlan
   - bottle:               DockerBottle handle
+  - provisioner:          DockerBottleProvisioner
   - backend:              DockerBottleBackend
 
 This file only re-exports the public names so
@@ -20,10 +21,12 @@ from .backend import DockerBottleBackend
 from .bottle import DockerBottle
 from .bottle_cleanup_plan import DockerBottleCleanupPlan
 from .bottle_plan import DockerBottlePlan
+from .provisioner import DockerBottleProvisioner
 
 __all__ = [
     "DockerBottle",
     "DockerBottleBackend",
     "DockerBottleCleanupPlan",
     "DockerBottlePlan",
+    "DockerBottleProvisioner",
 ]