Function load_skills 

pub fn load_skills(
    global_dir: Option<PathBuf>,
    project_dir: Option<PathBuf>,
) -> (Vec<Skill>, Vec<SkillLoadError>)

Load all skills from both global and project skill directories.

Each skill is a subdirectory containing a SKILL.md file. Project skills override global skills by name (same name field replaces). Only skills with enabled: true (or default) are included.

Returns a tuple of (loaded skills, load errors). Errors are non-fatal — the caller can log them and proceed with successfully loaded skills.