Add extension deletion reference to wix-app skill

[zivbwix]

Summary

• Adds skills/wix-app/references/EXTENSION_DELETION.md — a new reference file covering safe deletion of all 13 Wix CLI extension types
• Updates skills/wix-app/SKILL.md with a routing note, a Cross-Cutting References entry, and extended description triggers for deletion

What's in EXTENSION_DELETION.md

• Extension types reference table with default folder paths and per-type deviations
• Common deletion steps (delete folder + remove import/.use() from extensions.ts) with a before/after code example
• Extension-specific steps for the three deviating types:
  • Data Collection — two modes: single collection vs. entire extension deletion; CMS data warning
  • Backend API — file-only deletion, no extensions.ts entry exists
  • Site Plugin — extra step to delete the logo SVG from public/
• Validation steps (tsc --noEmit + wix build)
• Post-deletion requirements table (build+deploy for dashboard/backend; wix release for site extensions)
• Cost optimization tips
• Documentation links to the official delete section of each extension type's docs

Changes to SKILL.md

Three minimal additions to the existing file — no existing content altered:

1. description frontmatter — "Build, review, and delete" + deletion trigger keywords appended
2. Routing note after the opening paragraph directing deletion requests to EXTENSION_DELETION.md
3. New row in the Cross-Cutting References table

Test plan

• Verify EXTENSION_DELETION.md renders correctly on GitHub
• Confirm the routing note in SKILL.md links correctly to the new reference file
• Check that all documentation links in EXTENSION_DELETION.md resolve to the correct anchor sections on dev.wix.com