Cross-venue arbitrage spread between Polymarket and Limitless on a topic. POST {topic, limit?}. Pulls matching markets from Polymarket's public Gamma API and Limitless's REST search, pairs them by closest-price match, computes per-pair yes-mid spread + arbitrage direction. GA assembles a JOIN that single-venue passthroughs can't return. Naive pair-up — agent should confirm semantic match before sizing.