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/launch.py

@@ -208,19 +208,21 @@ def launch(
             compose_dump_logs, project, compose_file, compose_log_path(state_dir),
         )
 
-        # Step 8: provision. Unchanged — uses `docker exec` against
-        # the agent container by its known name.
-        prompt_path = provision(plan, plan.container_name)
+        # Step 8: provision. Create the bottle first so provisioners
+        # can use bottle.exec / bottle.cp_in; set the prompt path
+        # returned by provision_prompt after the fact.
+        bottle = DockerBottle(
+            plan.container_name,
+            teardown,
+            None,
+            agent_command=plan.agent_command,
+            agent_prompt_mode=plan.agent_prompt_mode,
+        )
+        bottle._prompt_path = provision(plan, bottle)
 
         # Step 9: yield. exec_agent continues to use `docker exec -it`
         # — the agent runs `sleep infinity` per the renderer's
         # service spec.
-        yield DockerBottle(
-            plan.container_name,
-            teardown,
-            prompt_path,
-            agent_command=plan.agent_command,
-            agent_prompt_mode=plan.agent_prompt_mode,
-        )
+        yield bottle
     finally:
         teardown()