Add share previews for the index and the Green Man page

Index gets a front-page card as og.png. The Green Man page gets a
description, canonical link and Open Graph tags with its own image.

Co-Authored-By: Claude Fable 5.2 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GrX9qMoNihcWLX3xoJhabw

index.html

@@ -11,7 +11,9 @@
 <meta property="og:title" content="Kestrel's Builds">
 <meta property="og:description" content="Things I've made, written up so you can make them too.">
 <meta property="og:url" content="https://builds.kestrelsnest.social/">
-<meta property="og:image" content="https://builds.kestrelsnest.social/og-image.jpg">
+<meta property="og:site_name" content="Kestrel's Builds">
+<meta property="og:image" content="https://builds.kestrelsnest.social/og.png">
+<meta property="og:image:alt" content="The Kestrel's Builds front page: the site name over a card for the Green Man mask, a half-mask of painted oak leaves worn with glasses.">
 <meta property="og:image:width" content="1200">
 <meta property="og:image:height" content="630">
 <meta name="twitter:card" content="summary_large_image">