docs(AGENTS): document mmdc for rendering mermaid diagrams to svg

AGENTS.md

@@ -5,6 +5,16 @@ Exercise repo: illustrating the transition from DB-backed domain classes
 Diagrams: `before-after-dto.mmd`, `after-dto.mmd` (source sketch:
 `before-after-dto-sketch.png`).
 
+## Rendering diagrams (mermaid → svg)
+
+Mermaid diagrams can be rendered to SVG with the `mmdc` CLI (mermaid-cli,
+installed via npm): `mmdc -i <input>` writes `<input>.svg` next to the source.
+
+```sh
+mmdc -i before-after-dto.mmd   # -> before-after-dto.svg
+mmdc -i after-dto.mmd          # -> after-dto.svg
+```
+
 ## Running dotnet
 
 dotnet SDK 10 is installed **via mise** (see `mise.toml`). It is NOT on PATH in