feat: initial Shopify Admin GraphQL MCP server

Read-only MCP server for the Shopify Admin GraphQL API (default version
2026-04). Exposes 13 curated tools — products, variants, metafields,
metafield definitions, collections, inventory/locations, shop settings —
plus a `shopify_graphql` escape hatch that rejects mutations.

- fetch-based GraphQL client with HTTP 429 and THROTTLED retry
- Custom-app access token auth (X-Shopify-Access-Token), no OAuth flow
- Cursor pagination on all list tools
- 17 passing unit tests (config, client, tool registration)
- tsdown build, oxlint/oxfmt, vitest — mirroring mcp-tastytrade conventions