Diagrams

Write D2 source and render it with the d2 CLI. If d2 is missing, ask the user to install it (sudo dnf install d2, brew install d2); don't fetch it any other way.

1. Write <name>.d2 where the user asked, otherwise in the project's tmp/.
2. Render: d2 --sketch <name>.d2 <name>.svg && rsvg-convert -z 2 <name>.svg -o <name>.png (hand-drawn look; drop --sketch if the user wants a clean one). Don't let d2 write PNG directly: older versions download a headless browser for it.
3. Open the PNG and check it: overlaps, cut or unreadable labels, tangled arrows. Fix the source and re-render, up to 3 rounds; --layout elk often untangles dense graphs.
4. Keep the .d2 next to the image so it can be edited later, and report both paths.