List all rule categories in the Email Playbook with a one-line description and page count. Categories are: structure (head/body container/header/body/footer), compatibility (Outlook MSO, RTL, responsive), production (Gmail clipping, dark mode, preheader, bulletproof buttons), ai-generation (constraints for AI emitters). For reusable components, use list_components instead — they live in a separate dimension and are not returned by get_playbook_rules.
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