Bump skills to 2026-04 schemas; add full instrumentation

- Update GQL schemas to 2026-04 (admin, storefront, customer, partner,
  payments-apps, functions)
- Add --client-version flag to search_docs.js and validate.js in all skills
- Forward model, clientName, clientVersion, source, skillVersion to
  instrumentation payload
- Fix malformed </system-instructions> tag in shopify-functions SKILL.md
- Add .gitignore to exclude node_modules

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>