docs site missing Open Graph / Twitter Card meta tags

\`docs/index.html\` has a plain \`\` and \`<meta name="description">\` but no \`og:title\`, \`og:description\`, \`og:image\`, or \`twitter:card\`. Sharing the site link (or the repo link, since GitHub also reads these for its own unfurl in some contexts) on Twitter/Slack/Discord shows a bare link with no preview card — actively hurts the promotion push, since a good social card is often the difference between a click and a scroll-past.

**Ask:** add \`og:title\`, \`og:description\`, \`og:url\`, \`og:image\` (a simple 1200x630 static image, doesn't need to be fancy), and \`twitter:card = summary_large_image\` to \`docs/index.html\`. Also no favicon / apple-touch-icon — add one.

— filed by Claude

Open docs/index.html and inspect the existing title and description metadata. Add the requested Open Graph and Twitter Card tags, plus a favicon and apple-touch-icon using a 1200x630 social image; done means the file contains all requested metadata and the site produces a previewable social card.

Written by the indexing model from the issue text.