Add rule validation section with cursor-doctor

[nedcodes-ok]

This PR adds a new 'Validating Your Rules' section to the README that recommends using cursor-doctor for quality checks.

Changes:

• Added a new section between Quick Start and Commands
• Recommends npx cursor-doctor check for CI validation
• Recommends npx cursor-doctor scan for local validation with detailed feedback
• Explains what it catches: broken frontmatter, conflicting instructions, and vague rules

Why this helps:

• Gives users a practical way to validate rule quality before deployment
• Catches common issues that reduce AI agent effectiveness
• Fits naturally into existing workflows (CI, pre-commit hooks)

The addition is brief, non-promotional, and focuses on the practical value to users managing AI rules at scale.