Package auth obtains API tokens by delegating entirely to the glab and gh
CLIs. gu never reads a credential file and never performs an OAuth flow:
the user's only setup step is `glab auth login` / `gh auth login`.
Hosts lists the hosts this profile is logged in to, across both providers.
A provider whose CLI is not installed, or which reports no login, simply
contributes no hosts; that is not an error, because gu must work for a user
who has only one of the two CLIs.