Create diagrams with D2

Create a diagram from the most recent interaction context using D2. Generate a PNG image and output it as a markdown image so it renders inline.

How to create a diagram

1. Extract or derive diagrammable data from the current context.
2. If the Emacs foreground color and background mode are not already known from a previous diagram in this session, query them:

   emacsclient --eval '(face-foreground (quote default))'
   emacsclient --eval '(frame-parameter nil (quote background-mode))'
   

   The first returns a hex color like "#eeffff". The second returns dark or light. Reuse both for all subsequent diagrams.
3. Write a D2 file to a temporary file using the foreground color.
4. Run D2 with --theme 200 if background mode is dark, or --theme 0 if light.
5. Output the result as a markdown image on its own line:

   ![description](/tmp/agent-diagram-XXXX.png)