fix: exclude goldrush+polymarket-resolution-delay from sitemap

[Flotapponnier]

Summary

Fixes 2 smoke-gate rollbacks:

• polymarket-resolution-delay: drafted in a worktree, never merged to main. Added to REMOVED_BENCH_SLUGS so sitemap excludes it even if stale Redis data surfaces it. 308 redirect in next.config.ts handles inbound links.
• goldrush: removed from indexing-freshness bench (PR feat: data-api report August 2026 #1758). Stale Redis data can keep it in getProviders() after removal while the page 404s (timing race). Added to REMOVED_PRODUCT_SLUGS and checked that set in sitemap-builder.ts validation loop.

Also adds REMOVED_PRODUCT_SLUGS import to sitemap-builder so future product page removals can use the same guard pattern without touching the builder.