Add design/ contract; archive prior design system as provenance

Establish keysat's durable, vendor-neutral design contract (the standards
/design backfill, document-as-is):

- design/DESIGN.md — nine-section brand brief distilled from the prior
  Claude Design system (navy-on-cream-paper identity, sovereignty-first
  voice, component + motion rules, do's/don'ts). Manrope is canonical
  display (the README's "Archivo" was a stale placeholder).
- design/tokens.tokens.json — W3C DTCG tokens from colors_and_type.css.
- design/brand/ — canonical palette.css + logo/mark assets.
- design/_imports/2026-06-16-claude-design-system/ — the original system,
  relocated as dated provenance (nothing imported it).
- AGENTS.md — add the Design line (read the contract before UI work);
  repoint the layout entry.
- ROADMAP.md — design-checker cleanup backlog (gold-as-fill + pill-radius
  blockers, the inline-token-copy consolidation, token gaps).

design/brand/bitcoin-mark.svg

@@ -0,0 +1,3 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32" fill="none">
+  <text x="16" y="24" text-anchor="middle" font-family="Archivo, Helvetica, sans-serif" font-weight="900" font-size="28" fill="#1E3A5F">₿</text>
+</svg>