Shipping feeds without a config file

Darkmown has no config file, so the site's public origin lives where the rest of its identity already does: the home page frontmatter. Add site_url to site/pages/index and the build emits sitemap.xml and rss.xml, reusing the home title and description as the RSS channel fields.

Every page picks up a <link rel="alternate" type="application/rss+xml"> so feed readers can autodiscover the feed, and robots.txt points crawlers at the sitemap. All build-time — not a byte of client JavaScript.