Add comparative design prototyping

[ariobarin]

Summary

• add Matt Pocock's prototype workflow as design-prototype`n- preserve the upstream UI and logic prototype playbooks
• rename only because current Codex already ships a system skill named prototypen- keep generic feasibility spikes on micro-experimentn

Evidence

• current Compass micro-experiment committed to one settings-page concept for a UI exploration
• design-prototype instead routed the same request to three structurally different variants on the existing route with ?variant= and a dev-only switcher
• generic runtime spike still selected micro-experiment`n- git diff --check, pwsh tests, and Windows PowerShell 5.1 tests pass