Function resolve_repos_config_path 

pub fn resolve_repos_config_path() -> Option<PathBuf>

What: Locate the repos.conf file for third-party repository definitions (TOML).

Inputs:

• None (reads HOME / XDG_CONFIG_HOME).

Output:

• Some(PathBuf) when a candidate file exists; None otherwise.

Details:

• Checks $HOME/.config/pacsea/repos.conf then XDG_CONFIG_HOME/pacsea/repos.conf.
• Does not fall back to legacy pacsea.conf (repos live only in the split layout).