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

Phase 2 of 3, derived from requirements.md. Compact by tier.

Architecture

Two checked-in artifacts under a new docs/assets/ home (repo-level README images had none; per-spec images live under docs/specs/<id>/design/):

• docs/assets/the-loop-workflow.excalidraw — the scene source, generated programmatically so geometry stays consistent across regenerations.
• docs/assets/the-loop-workflow.svg — the embed the README references, produced by Excalidraw's own exportToSvg (headless Chromium, scratchpad-only tooling; nothing enters the repo): Virgil font embedded as a data URI, exportEmbedScene on so the SVG itself reopens in excalidraw.com.

GitHub cannot render .excalidraw files, hence the SVG indirection. The diagram mirrors the mermaid original 1:1: ticket → (brainstorm) → requirements → design → tasks inside a "specified and reviewed" zone; implement/self-check → self/critic review → evidence inside an "executed autonomously" zone; complete → learn; phase labels rendered as gray captions under each node.

userInteraction.diagramFormat: mermaid is untouched: it governs diagrams the harness produces (specs, PR briefings). This is an explicit operator request scoped to the README hero diagram.

Security design

The requirements name no trust boundary and no abuse case; this design keeps it that way. The one considered vector — SVG script execution — is closed by construction: the file comes from Excalidraw's exporter (shapes, paths, an embedded font and the scene payload; no <script>, no event handlers), and GitHub additionally serves README images through its sanitizing image proxy. Fail-closed check: the exported SVG was grepped for scripting before commit.

Testing strategy

Docs-only change, no code paths: verification is markdownlint on the changed markdown (same hook locally and in CI), a JSON validity check of the scene, a headless-Chromium screenshot of the SVG reviewed visually against the mermaid original, and a round-trip of both files through excalidraw.com.