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/prompt.py

@@ -7,36 +7,26 @@ actually has a prompt — the return value signals which case."""
 from __future__ import annotations
 
 import os
-import subprocess
 
+from ... import Bottle
 from ..bottle_plan import DockerBottlePlan
 
 
-def provision_prompt(plan: DockerBottlePlan, target: str) -> str | None:
+def provision_prompt(plan: DockerBottlePlan, bottle: Bottle) -> str | None:
     """Copy the prompt file into the container, fix ownership/mode.
     Returns the in-container path if the agent has a non-empty
     prompt (drives --append-system-prompt-file), else None. The
     file is copied either way so the path always exists."""
-    container = target
     container_home = os.environ.get("BOT_BOTTLE_CONTAINER_HOME", "/home/node")
     in_container_prompt_path = f"{container_home}/.bot-bottle-prompt.txt"
 
-    subprocess.run(
-        ["docker", "cp", str(plan.prompt_file), f"{container}:{in_container_prompt_path}"],
-        stdout=subprocess.DEVNULL,
-        check=True,
-    )
+    bottle.cp_in(str(plan.prompt_file), in_container_prompt_path)
     # `docker cp` preserves host UID; re-own/mode as root so node
     # can read its own mode-600 prompt regardless of host UID.
-    subprocess.run(
-        ["docker", "exec", "-u", "0", container, "chown", "node:node", in_container_prompt_path],
-        stdout=subprocess.DEVNULL,
-        check=True,
-    )
-    subprocess.run(
-        ["docker", "exec", "-u", "0", container, "chmod", "600", in_container_prompt_path],
-        stdout=subprocess.DEVNULL,
-        check=True,
+    bottle.exec(
+        f"chown node:node {in_container_prompt_path} && "
+        f"chmod 600 {in_container_prompt_path}",
+        user="root",
     )
 
     agent = plan.spec.manifest.agents[plan.spec.agent_name]