refactor(cli): fold the chatgpt sign-in into docker agent setup

The auth command group (login/logout/status) added a whole command tree
for a single provider. The setup wizard already runs the browser sign-in
when chatgpt is picked, so it becomes the only entry point:

- remove `docker agent auth` and its tests
- point every sign-in hint (doctor, missing-env errors, token refresh,
  auto-selection) at `docker agent setup`
- document sign-in status via doctor and sign-out as removing
  chatgpt-auth.json

Author: Sayt-0