feat: add social preview image (opendecree/decree#147)

1280×640 PNG for GitHub link unfurls on X/LinkedIn/Slack.
SVG source committed alongside; re-render with:

  rsvg-convert assets/social-preview.svg -w 1280 -h 640 \
    -o assets/social-preview.png

Upload via Settings → Social preview.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: zeevdr