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/supervise.py

@@ -18,10 +18,9 @@ sidecar that isn't running.
 
 from __future__ import annotations
 
-import subprocess
-
 from ....log import info, warn
 from ....supervise import SUPERVISE_HOSTNAME, SUPERVISE_PORT
+from ... import Bottle
 from ..bottle_plan import DockerBottlePlan
 
 
@@ -32,7 +31,7 @@ def supervise_mcp_url() -> str:
     return f"http://{SUPERVISE_HOSTNAME}:{SUPERVISE_PORT}/"
 
 
-def provision_supervise(plan: DockerBottlePlan, target: str) -> None:
+def provision_supervise(plan: DockerBottlePlan, bottle: Bottle) -> None:
     """Run `claude mcp add` inside the agent container to register
     the supervise sidecar in claude-code's user config. No-op when
     bottle.supervise is False.
@@ -43,16 +42,11 @@ def provision_supervise(plan: DockerBottlePlan, target: str) -> None:
     if plan.supervise_plan is None:
         return
     url = supervise_mcp_url()
-    argv = [
-        "docker", "exec", "-u", "node", target,
-        "claude", "mcp", "add",
-        "--scope", "user",
-        "--transport", "http",
-        _SUPERVISE_MCP_NAME,
-        url,
-    ]
     info(f"registering supervise MCP server in agent claude config → {url}")
-    r = subprocess.run(argv, capture_output=True, text=True, check=False)
+    r = bottle.exec(
+        f"claude mcp add --scope user --transport http {_SUPERVISE_MCP_NAME} {url}",
+        user="node",
+    )
     if r.returncode != 0:
         warn(
             f"`claude mcp add supervise` failed (exit {r.returncode}): "