blume@1.4.2

fdfd1fe: Advertise agent discovery in every page’s head, not just the homepage header

An agent that enters the site on a deep page — a search result, a shared link — never sees the homepage-only Link response header, so it had no path to agent-readability.json. Every rendered page now carries the discovery links in its HTML head: describedby links to agent-readability.json and llms.txt, plus a text/markdown alternate pointing at the page’s own raw-Markdown mirror. Because the links travel with the prerendered HTML, they also work on hosts where Blume can’t set response headers at all (GitHub Pages, S3).
5f735ae: Raise minimum versions of astro, mermaid, dompurify, undici, js-yaml, and @astrojs/vercel to pull in upstream security fixes