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

README.md

@@ -10,7 +10,7 @@ A static site with no build step. Each build is a self-contained folder; the top
 
 ```
 index.html        - Site index: one card per build
-og-image.jpg      - OpenGraph image for the index (1200 x 630)
+og.png            - OpenGraph image for the index (1200 x 630); each build keeps its own og-image.jpg
 deploy            - Deployment script (rsync to server)
 greenman/         - Green Man mask
   index.html      - The build and painting guides as one page