Add Shopify agent skills for Claude Code, Cursor, and Claude Desktop

Adds 15 agent skills under skills/ — the conventional directory for
`npx skills add Shopify/shopify-ai-toolkit`. Each skill bundles a
SKILL.md with agent instructions and mandatory search/validate steps,
pre-compiled scripts (search_docs.js, validate.js), and compressed
schema assets where applicable.

Skills included:
- shopify-admin (Admin GraphQL API)
- shopify-storefront-graphql (Storefront API)
- shopify-customer (Customer Account API)
- shopify-partner (Partner API)
- shopify-payments-apps (Payments Apps API)
- shopify-functions (Shopify Functions)
- shopify-hydrogen (Hydrogen / Remix)
- shopify-liquid (Liquid / Theme development)
- shopify-polaris-admin-extensions (Admin UI Extensions)
- shopify-polaris-checkout-extensions (Checkout UI Extensions)
- shopify-polaris-customer-account-extensions (Customer Account UI Extensions)
- shopify-polaris-app-home (App Home)
- shopify-pos-ui (POS UI Extensions)
- shopify-custom-data (Metafields / Metaobjects)
- shopify-dev (General Shopify dev)

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