Tasks: Replace the README workflow mermaid diagram with an Excalidraw diagram

Phase 3 of 3, derived from requirements.md and design.md.

Task list

• [x] T1 — Author the workflow scene (generator script → .excalidraw JSON), content-matched to the mermaid original (req 2).
• [x] T2 — Export docs/assets/the-loop-workflow.svg via Excalidraw's exportToSvg with fonts embedded and exportEmbedScene on (req 3); verify no scripting in the output.
• [x] T3 — Replace the README mermaid block with the SVG embed plus a pointer to the editable sources (req 1).
• [x] T4 — Verify: markdownlint on changed markdown, scene JSON validity, visual check of the rendered SVG against the mermaid original.