Add site extractors for two prediction market platforms using their
public REST APIs (no browser/Playwright dependency needed).
Polymarket: search via Gamma API, get events/markets by slug/ID/URL.
Handles inconsistent JSON encoding (stringified arrays).
Kalshi: get markets/events by ticker/URL, search via client-side
filtering of open events (no text search API available).
Both include CLI tools and comprehensive test suites using httptest.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
