Function write_learned_skill 

pub fn write_learned_skill(
    skills_dir: &Path,
    candidate: &CandidateProcedure,
    md_content: &str,
) -> Result<PathBuf>

Write a learned skill to disk under {skills_dir}/learned/.

Uses atomic write (write to .tmp then rename) so that a concurrent SkillLoader never observes a partially-written .md file.