Function run_external_learning 

pub async fn run_external_learning<L: EvolutionLlm>(
    chat_root: &Path,
    llm: &L,
    model: &str,
    txn_id: &str,
) -> Result<Vec<(String, String)>>

Run external learning cycle. Returns (change_type, id) pairs for the changelog.

Gated by SKILLLITE_EXTERNAL_LEARNING=1. If not enabled, returns Ok(empty). Opens its own SQLite connection so the future is Send.