Add design/ contract extracted from the as-built UI

Inventory the as-built recaps.cc look and distill it into a durable,
vendor-neutral design contract: design/DESIGN.md (nine-section brand
brief) + design/tokens.tokens.json (W3C DTCG tokens), plus brand icon
and provenance notes. Canonical calls reconciled with the owner:
indigo #818cf8 as the single interactive accent, purple #a855f7 for
premium only, the #0a0e1a->#111827->#0f172a surface ladder, and a
normalized type scale. Wire the AGENTS.md Design line and record the
contract-vs-code drift as a cleanup backlog in ROADMAP.md.

design/brand/README.md

@@ -0,0 +1,17 @@
+# Brand assets
+
+- **`icon.png`** — the app icon (1024×1024). A rounded-square (squircle) on the
+  dark navy field, with a **play-triangle filled by a blue→purple gradient** over
+  four light "transcript" lines. Used as the PWA icon (`manifest.json` →
+  `/assets/icon.png`, also `assets/icon.png` / `icon.png` at repo root) and the
+  auth-screen logo. The gradient's blue end and purple end are the source of the
+  app's blue (info) and purple (premium) accents; the indigo `#818cf8` interactive
+  accent sits between them.
+
+**Fonts:** none shipped — Recap uses the system font stack (`-apple-system, …`)
+and a system monospace stack. No web-font files to vendor here.
+
+**Palette:** the machine-readable source of truth is `../tokens.tokens.json`. No
+generated `palette.css` yet — styles live inline in `public/index.html`;
+consolidating them behind CSS custom properties is tracked as cleanup in the
+repo `ROADMAP.md`.