refactor(backend): fold BottleProvisioner back into BottleBackend

BottleProvisioner had no independent identity — no state, only one
caller, never selected, never crossed a method boundary as data. It
was a method dressed up as a class. Reverting that turn:

  - BottleBackend gains an abstract provision(plan, target).
  - DockerBottleBackend.provision absorbs the body that lived on
    DockerBottleProvisioner.
  - backend/docker/provisioner.py deleted.
  - BottleProvisioner ABC removed from backend/__init__.py.
  - launch now calls self.provision(plan, container) directly.

Net: -1 file, -1 class, -1 ABC. Same behavior; tests pass.

claude_bottle/backend/docker/__init__.py

@@ -7,7 +7,6 @@ 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
@@ -21,12 +20,10 @@ 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",
 ]