feat(wc-catalog-expansion): Stage 9C — WooCommerce catalog expansion

- Add ProductCategory domain model (id, name, slug, parentId, count) with copyWith, equality, toString
- Add ProductImage domain model (id, url, alt, position) with copyWith, equality
- Extend ProductPublishingRepository contract: createProduct, deleteProduct, updateProductImages, getProductImages, getCategories, createCategory, updateCategory, deleteCategory
- Implement all 8 new methods in FakeProductPublishingRepository with seed categories
- Implement all 8 new methods in WordPressProductPublishingRepository
- Add WooCommerceApiClient methods: createProduct, deleteProduct, getCategories (paginated), createCategory, updateCategory, deleteCategory, getProductImages, setProductImages
- Add use cases: CreateProduct, DeleteProduct, GetCategories, CreateCategory, UpdateProductImages
- Update feature_wordpress.dart barrel exports
- Update test stubs in dashboard_controller_test.dart and product_publishing_page_test.dart
- Add 69 new tests in wc_catalog_expansion_test.dart (388 total feature_wordpress, 24 kell_web — all passing)
- Update master_development_brief.md and build_execution_tracker.md