Feature: add CSS custom properties export (--format css-vars) for native CSS consumers

[dl-alexandre]

Add a CLI export target for CSS variables, for example:

npx @google/design.md export --format css-vars DESIGN.md > design.tokens.css

Expected output

Generate a :root block with stable, semantic custom properties derived from tokens, e.g.:

:root {
  --color-primary: #1A1C1E;
  --color-secondary: #6C7278;
  --spacing-sm: 8px;
  --spacing-md: 16px;
  --rounded-sm: 4px;
  --rounded-md: 8px;
}

Optionally support:

• --selector :root
• --prefix ds
• dark-theme/media-query variants later if the spec grows there

Why this helps

• makes DESIGN.md more useful outside Tailwind ecosystems
• gives coding agents a direct artifact they can apply in native CSS codebases
• improves adoption for frameworks that want CSS variables as the implementation boundary
• complements DTCG export without requiring downstream token tooling

[seonghobae]

Current roadmap linkage refresh: issue #15 remains a direct follow-up under #29 (Token interoperability roadmap). The current canonical sequence is #30 (workspace/build reliability) first, then export-surface delivery such as #15 and #19, with #28 after those and #27 as a cross-cutting spec dependency where needed.