Diagram architecture

Create or update ARCHITECTURE.md for a task with Mermaid diagrams.

Trigger: "Diagram architecture" or /spec.md.diagram-architecture

Resolves the task in context (drafts/ or active/; asks for slug if ambiguous). Reads APPROACH.md and affected code paths, then writes ARCHITECTURE.md in the task folder with one or more mermaid blocks, a legend, and links to real files.

ARCHITECTURE.md is optional and generated on demand — it is not part of the core A-B-C set. Use /spec.md.create-task for planning; run this command when a visual architecture artifact helps review.