Create diagrams with PlantUML

Create a diagram from the most recent interaction context using PlantUML. Generate a PNG image with a transparent background 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 is not already known from a previous diagram in this session, query it:

   emacsclient --eval '
   (face-foreground (quote default))'
   

   This returns a hex color like "#eeffff". Reuse it for all subsequent diagrams.
3. Write a PlantUML file to a temporary file using that color.
4. Run PlantUML on the file.
5. Output the result as a markdown image on its own line:

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