refactor(backend): pass Bottle to provisioners instead of target string

Closes #178.

The backend provision functions now receive a Bottle handle with
exec / cp_in methods instead of a raw target string. Provisioner
modules use bottle.exec and bottle.cp_in in place of inlined
subprocess.run(["docker", "exec"/"cp", ...]) and direct
_smolvm.machine_cp / machine_exec calls. This decouples the
provisioners from backend-specific runtime primitives so future
refactors (e.g. the supervise rework) can swap the bottle's exec
implementation without touching every provisioner.

Each launch.py constructs the Bottle handle before calling
provision so it can be passed in; provision_prompt's return value
is wired back onto the bottle's prompt path attribute after the
fact.

bot_bottle/backend/docker/provision/__init__.py

@@ -1,7 +1,7 @@
 """Per-provisioner modules for the Docker backend.
 
 Each module exports one top-level function:
-    provision_<thing>(plan: DockerBottlePlan, target: str) -> ...
+    provision_<thing>(plan: DockerBottlePlan, bottle: Bottle) -> ...
 
 `DockerBottleBackend.provision_*` methods delegate to these. The
 abstract `BottleBackend.provision_*` surface is unchanged; this