Discoverability: SEO, feeds, Markdown mirrors, llms.txt

[Artic0din]

Part of #1.

Scope

• Next.js Metadata API on every route: titles, descriptions, canonical URLs, Open Graph and Twitter cards.
• Generated Open Graph images for posts and pages.
• sitemap.xml, robots.txt, RSS feed, JSON-LD (WebSite, BlogPosting).
• Favicon and web manifest.
• Markdown mirrors: /index.md, /posts.md, /posts/<slug>.md and /llms.txt, generated at build time for AI readers.
• Tests for the feed, sitemap and Markdown mirror output.

Done when

• The pull request checks pass.
• On the live site, the feed validates, the sitemap lists every published page, and each Markdown mirror returns text/markdown.