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/__init__.py

@@ -104,6 +104,22 @@ class Bottle(ABC):
     def close(self) -> None: ...
 
 
+class BottleProvisioner(ABC):
+    """Copies host-side files (prompt, skills, SSH keys, .git) into a
+    running bottle after the container/machine is up. Owned by a
+    BottleBackend; called from its launch step before yielding the
+    Bottle handle."""
+
+    @abstractmethod
+    def provision(self, plan: BottlePlan, target: str) -> str | None:
+        """Provision the running bottle described by `plan`. `target`
+        identifies the running instance in backend-specific terms
+        (Docker: resolved container name; fly: machine id). Returns the
+        in-container prompt path if a prompt was provisioned, else
+        None — the Bottle handle uses it to decide whether to add
+        --append-system-prompt-file to claude's argv."""
+
+
 class BottleBackend(ABC):
     """Abstract base for selectable bottle backends. Concrete subclasses
     (e.g. DockerBottleBackend) own their own prepare/launch impls.
@@ -162,6 +178,7 @@ __all__ = [
     "BottleBackend",
     "BottleCleanupPlan",
     "BottlePlan",
+    "BottleProvisioner",
     "BottleSpec",
     "get_bottle_backend",
 ]