explain-diff-myst

Use when asked to explain a code change, diff, commit range, branch, or PR to a human reader as a MyST Markdown document with mermaid diagrams, callouts, and a dropdown-based quiz. Trigger on "explain this diff/PR/branch", "write up this change", "onboarding doc for this change", or a request for a rendered/publishable Markdown explanation. Not for writing commit messages, PR descriptions, changelogs, or release notes, and not for reviewing a change for defects.