fix(ci): smoke against deploy URL to avoid CDN cache false rollbacks

[Flotapponnier]

Summary

• Change prod-deploy smoke test to run against the deployment URL (steps.deploy.outputs.url) instead of https://openchainbench.com
• /sitemap.xml has s-maxage=3600 — CDN serves the old cached sitemap for up to 1h after each deploy, causing correct deployments to fail smoke and rollback
• The deploy URL bypasses CDN cache and tests the actual newly deployed code

Root cause

After vercel deploy --prod switches the prod alias, the CDN still serves the previous deployment's cached /sitemap.xml. The smoke was seeing the old 881-URL sitemap (with noindex/404 entries) instead of the freshly generated correct one, triggering rollbacks of perfectly valid deployments.

Test plan

• Next prod deploy: smoke runs against deployment URL, gets fresh sitemap, passes