Align admin GraphQL operations and codegen with GitStore resource schema

[juliuskrah]

Parent initiative: #287

Summary

Align commerce admin GraphQL operations, fragments, and generated types with the GitStore resource schema used by catalog operations and operator console workflows.

Acceptance criteria

• Every committed admin query and mutation is represented in codegen and compiles against the current GitStore schema.
• Shared fragments cover namespace, repository, store context, catalog resources, products, ProductVariant data, collections, categories, and API errors.
• Generated types are consumed by admin views instead of handwritten response shapes.
• Loading, GraphQL error, validation error, and empty-result mappings are documented in code and covered by unit tests.
• npm run codegen is required for the admin package and fails on stale operations.

Architecture note

gitstore-admin remains an optional add-on and consumes gitstore-api; it must not talk directly to gitstore-git-service.