feat: add Warm Shelf headless demo

[hanag-wix]

Summary

• Adds Warm Shelf — a bookstore-café with new-title shop, free book-crossing shelf, author events, and an interactive Ex Libris stamp studio
• Live site: https://soggz4-warm-shelf-01f52b97-tetianast.wix-site-host.com/
• Source: https://github.com/TetianaStashchenko/warm-shelf

Wix integrations

• Stores + eCommerce — add to cart → custom /cart → hosted Wix checkout
• Bookings + eCommerce — paid craft sessions → hosted checkout → /booking-confirmation
• Events — free RSVP
• CMS — BookCrossingShelf collection for the community library
• Forms — shelf book reservations

Cleanup applied

• Removed wix.config.json (added wix.config.example.json + .gitignore entry)
• Excluded .agents/, .vscode/, skills-lock.json
• Project-specific README with npm create @wix/new@latest init
• Added to root README projects table

Test plan

• cd warm-shelf && npm install && npm run build succeeds
• Shop add-to-cart → /cart → checkout works after linking own Wix site
• Bookings flow redirects to hosted checkout for paid services