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/ca.py

@@ -31,33 +31,21 @@ stage dir; nothing in the agent ever sees it."""
 
 from __future__ import annotations
 
-import subprocess
-
+from ... import Bottle
 from ...util import AGENT_CA_PATH, log_ca_fingerprint, select_ca_cert
 from ..bottle_plan import DockerBottlePlan
 
 
-def provision_ca(plan: DockerBottlePlan, target: str) -> None:
+def provision_ca(plan: DockerBottlePlan, bottle: Bottle) -> None:
     """Copy the agent-facing CA cert into the agent, rebuild the
     trust bundle, emit a one-line fingerprint log. Called from
     `BottleBackend.provision` after the agent container is up."""
-    container = target
     cert_host_path, label = select_ca_cert(plan.egress_plan, plan.proxy_plan)
 
-    subprocess.run(
-        ["docker", "cp", str(cert_host_path), f"{container}:{AGENT_CA_PATH}"],
-        stdout=subprocess.DEVNULL,
-        check=True,
-    )
-    subprocess.run(
-        ["docker", "exec", "-u", "0", container, "chmod", "644", AGENT_CA_PATH],
-        stdout=subprocess.DEVNULL,
-        check=True,
-    )
-    subprocess.run(
-        ["docker", "exec", "-u", "0", container, "update-ca-certificates"],
-        stdout=subprocess.DEVNULL,
-        check=True,
+    bottle.cp_in(str(cert_host_path), AGENT_CA_PATH)
+    bottle.exec(
+        f"chmod 644 {AGENT_CA_PATH} && update-ca-certificates",
+        user="root",
     )
 
     log_ca_fingerprint(cert_host_path, label)