bt setup --json does nothing

3694f09be9103827b421ca654bc490bb708b3937 broke `bt setup --json`, now it does nothing.

Works on previous commit d019902b955edebc03cfa2da8096b485c2a88620:
```
bt setup --json
✔ Select coding agent · codex
{
  "scope": "global",
  "selected_agents": [
    "codex"
  ],
... rest of the json
```

Although I'm not sure if this command really has any utility so maybe it's not an issue that it broke.