feat: add App Store Readiness skill

[andyhorn]

Description

Add a skill to evaluate an app's readiness/compliance with the Apple and Google app store policies; call out any Flutter-specific topics.

Requirements

• All CI/CD checks are passing.
• There is no drop in the test coverage percentage.

Additional Context

GitHub has a skill available on skills.sh: https://skills.sh/github/awesome-copilot/apple-appstore-reviewer