feat(design): add agent-consumable design surface

[bntvllnt]

Summary

• Added packages/design/tokens.json with schema/docs and mirrored it at /r/design.json.
• Added /design and /design.txt to expose the root design guide through the registry site, with TOC, metadata, sitemap, nav/footer, and llms integration.
• Added Storybook design-token docs and design-rule references in AGENTS, CONTRIBUTING, and the PR template.

Closes #250

Validation

• pnpm -F @vllnt/ui lint PASS
• pnpm -F @vllnt/ui exec tsc --noEmit --project tsconfig.build.json PASS
• pnpm -F @vllnt/ui-registry exec eslint app/design/page.tsx app/design.txt/route.ts app/r/design.json/route.ts app/llms.txt/route.ts app/llms-full.txt/route.ts app/sitemap.ts components/header/header.tsx components/footer/footer.tsx lib/design-guide.ts PASS
• pnpm -F @vllnt/ui-registry exec tsc --noEmit PASS
• pnpm build PASS
• pnpm test:once PASS (216 files, 1215 tests)
• pnpm -F @vllnt/ui build-storybook PASS
• git diff --check PASS

Note: full pnpm -F @vllnt/ui-registry lint still hits the existing jsx-a11y/label-has-associated-control/minimatch runtime error in app/report/report-bug-form.tsx, outside this PR.

Design Deviations

None.

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated (UTC)
storybook	Ready	Preview, Comment	May 12, 2026 5:31pm
ui.vllnt.ai	Ready	Preview, Comment	May 12, 2026 5:31pm