Function promote_learning_candidate 

pub async fn promote_learning_candidate(
    state: &AppState,
    chat_id: i64,
    candidate_id: &str,
) -> Result<String, String>

Materialize a reviewed external-learning candidate through the same version registry used by other agent-created skills. Existing skill names are rejected: improvements to an active skill must use comparative reflection and shadow evidence instead of bypassing its baseline.