Function resolve_models_dir 

pub fn resolve_models_dir(
    explicit: Option<&str>,
) -> Result<ModelsDirResolution, PathError>

Resolve the models directory from an explicit override, env var, or default.

Resolution order:

1. Explicit path provided by caller (highest priority)
2. GGLIB_MODELS_DIR environment variable
3. Default models directory (~/.local/share/llama_models)