Function load_default_paths 

pub fn load_default_paths(
    registry: &ConfigRegistry,
    workspace_root: Option<&Path>,
    structural_prefixes: &[&str],
) -> LoadOutcome

Load both default paths in standard precedence order: user first, then project. Missing files are silent (no message). structural_prefixes lists dotted prefixes the caller wants to handle itself (e.g. &["completion.per-language", "plugin"]).