feat: branded social-preview cards for shared repos

[JustMaris]

Type-only 1280×640 social preview cards so github.com/drumandbytes/* links unfurl as a consistent branded set instead of GitHub's auto-generated counter card.

• profile/social/generate.sh — reproducible generator (ImageMagick), tagline table inline
• profile/social/*.png — cards for eraser, f1-walk, road-conditions-ee, opentofu-updater-action, argocd-gitops-updater-action
• profile/social/README.md — which file goes to which repo, and how to upload

Design: dark ground, orange accent bar (#FF6633, the site's --ghost-accent-color), repo name + one-line tagline, drumandbytes.com wordmark. No imagery — minimal by choice.

Manual step after merge: social preview is a per-repo GitHub setting with no API — each PNG has to be uploaded at <repo> → Settings → General → Social preview. The nordvpn / pve-metrics-exporter infra repos are left on GitHub's default card (people share their GHCR package, not the repo). The org page itself can't have a custom card.

🤖 Generated with Claude Code

[JustMaris]

Not merging — the images have to be uploaded to each repo's settings by hand anyway, so keeping them version-controlled here isn't worth the repo weight. Kept locally if needed later.