pub fn discover_config_path() -> Option<PathBuf>
Discover the default XDG config file path.
Returns $XDG_CONFIG_HOME/agent-exec/config.toml if XDG_CONFIG_HOME is set, otherwise returns ~/.config/agent-exec/config.toml.
$XDG_CONFIG_HOME/agent-exec/config.toml
XDG_CONFIG_HOME
~/.config/agent-exec/config.toml