Add 1280x640 social preview PNG and upload to repo settings

[bordeauxred]

Why

GitHub does not consume `social-preview.html` for OG cards. When the AI Tinkerers newsletter (and downstream X/LinkedIn shares) link the repo, the Open Graph card will fall back to the default GitHub chrome unless a PNG is explicitly uploaded.

What

• Render `social-preview.html` to `social-preview.png` at 1280x640. Quickest paths:
  • Open in browser sized 1280x640, screenshot, save as PNG
  • `npx --yes capture-website-cli social-preview.html --width=1280 --height=640 --output=social-preview.png`
• Commit the PNG to the repo root.
• Upload via `https://github.com/aganthos/clawloop\` → Settings → Social preview → Edit → upload PNG. (Manual UI step.)
• Verify via `https://opengraph.dev/?url=https://github.com/aganthos/clawloop\` — preview card should show the rendered image.
• Do this ≥6 hours before newsletter send — GitHub OG responses can cache aggressively on Twitter/X and LinkedIn.

Acceptance

`opengraph.dev` shows the uploaded PNG for the repo URL.

Why this matters

Single most visible launch artifact for share-card audiences who never click through.