Add SKU resource to `core` and `hooks` packages

[acasazza]

Summary

Add support for the SKU resource in the @commercelayer/core and @commercelayer/hooks packages, following the same pattern established by the prices resource.

Motivation

The prices resource is already implemented across both packages (core/src/prices/ and hooks/src/prices/). SKUs are a fundamental resource in Commerce Layer and should be available with the same level of abstraction for consumers of the v5 packages.

Tasks

packages/core

• Add src/skus/getSku.ts — fetch a single SKU by ID
• Add src/skus/getSkus.ts — fetch a list of SKUs (with optional filters)
• Add src/skus/index.ts — re-export SKU functions
• Add src/skus/getSku.spec.ts / getSkus.spec.ts — integration tests using coreTest fixture
• Export SKU functions from src/index.ts

packages/hooks

• Add src/skus/useSkus.ts — React hook wrapping the core getSkus function (using SWR, mirroring usePrices)
• Add src/skus/useSku.ts — React hook for fetching a single SKU
• Add src/skus/index.ts — re-export SKU hooks
• Add src/skus/useSkus.test.ts — unit tests
• Export SKU hooks from src/index.ts

Reference

• Existing implementation to follow: packages/core/src/prices/ and packages/hooks/src/prices/
• SDK resource: CommerceLayerClient.skus