📚 docs(document): add SKUs story and upgrade Storybook to v10

[acasazza]

Summary

Add the first SKU documentation story to the packages/document Storybook and upgrade Storybook to the latest version.

Changes

• Upgrade all @storybook/* packages to ^10.3.4 (from mixed v8/v10)
• Add @storybook/addon-mcp with MCP server configuration (.mcp.json)
• Add packages/document/src/stories/skus/001.skus.mdx — docs for all 5 SKU components (SkusContainer, Skus, SkuField, SkuListsContainer, SkuList)
• Add packages/document/src/stories/skus/skus.stories.tsx — 3 interactive visual-test stories backed by MSW mocks
• Add MSW mock handlers for /api/skus and /api/sku_lists/:id