docs(markdown): avoid “Executive Summary” headings

- Prefer “TL;DR” or “Summary” to reduce AI-tone signals

Author: d-padmanabhan

rules/800-markdown.mdc

@@ -566,6 +566,7 @@ Emojis MAY be used in:
 - Add horizontal rules (`---`) to separate sections
 - Maintain consistent formatting
 - Ensure mobile-friendly layouts
+- Avoid headings like **"Executive Summary"** (reads AI-generated); prefer **"TL;DR"** or **"Summary"**
 
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