Create a site by downloading a single page from a known artifact URL.

The shortcut past copy-and-paste for a page an AI chat built: send the artifact's public address and the fetched HTML becomes the site's index.html. Only hosts that serve an artifact as a plain document can be fetched — see lib/artifact-sources.ts for the list — and a chat's own share link is refused with the reason, because it renders the conversation rather than the page inside it. One file, 5 MB; use sites.create for anything with assets.