[NO JIRA] backpack-code-review-checklist SKILL

[GC Zhu (gc-skyscanner)]

This PR based on #4297
Remember to include the following changes:Updates the backpack-code-review-checklist Claude Code skill (v1.2.0 → v1.3.0) to include a new checklist rule and guidance around how variant/enum props should be typed.

• Ensure the PR title includes the name of the component you are changing so it's clear in the release notes for consumers of the changes in the version e.g [Clover-123][BpkButton] Updating the colour
• README.md (If you have created a new component)
• Component README.md
• Tests
• Accessibility tests
  • The following checks were performed:
    • Ability to navigate using a keyboard only
    • Zoom functionality (Deque University explanation):
      • The page SHOULD be functional AND readable when only the text is magnified to 200% of its initial size
      • Pages must reflow as zoom increases up to 400% so that content continues to be presented in only one column i.e. Content MUST NOT require scrolling in two directions (both vertically and horizontally)
    • Ability to navigate using a screen reader only
• Storybook examples created/updated
• For breaking changes or deprecating components/properties, migration guides added to the description of the PR. If the guide has large changes, consider creating a new Markdown page inside the component's docs folder and link it here

[skyscanner-backpack-bot]

Visit https://backpack.github.io/storybook-prs/4309 to see this build running in a browser.