Function agent_gh_config_dir 

pub fn agent_gh_config_dir(
    config_home: &Path,
    agent_slug: Option<&str>,
) -> PathBuf

<config_home>/gh-<slug>, the same location agent.open has always used.

The slug is reduced to [a-z0-9_-] so it can never name a path outside config_home — this directory is one clear_gh_login deletes from.