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

Phase 1 of 3 (requirements → design → tasks). Tier 1–2 (trivial docs change, autonomy.tiers → autonomous-complete): this spec is deliberately compact. The requirement is the operator's direct, unambiguous instruction — "create an excalidraw diagram instead of the mermaid diagram for the workflow depicted in README.md" — and they asked to iterate on the result through the PR, which stands as the phase approval (issue #150).

Introduction

The README's "The loop, in one line" section depicts the workflow as a mermaid flowchart TD. The operator wants that hero diagram to be a hand-drawn-style Excalidraw diagram instead, without losing any of the information the mermaid version carries.

Requirements

User story: As a reader of the README, I want the workflow diagram as an Excalidraw drawing, so the project's hero visual is friendlier while carrying the same information.

Acceptance criteria (EARS):

1. WHEN the README is viewed on GitHub THEN the system SHALL render an Excalidraw workflow diagram in place of the mermaid flowchart TD block.
2. WHEN the diagram is compared with the removed mermaid original THEN the system SHALL preserve every node, edge, edge label ("convert", "human review"), the two zone groupings, and the loop:<phase> labels.
3. WHEN a contributor wants to edit the diagram THEN the system SHALL provide a checked-in editable source that opens at excalidraw.com.

Security considerations

Threat-model-lite: no untrusted actors, no trust boundary is created or moved, and no abuse case applies — the change is a static image plus markdown in a public README. The only vector considered is the embedded image itself: SVG can carry scripts, so the export must contain no scripting (verified in design). No new attack surface.