feat: add CSS custom properties export format (--format css-vars)

[mvanhorn]

Summary

Adds a css-vars export target so design.md export --format css-vars DESIGN.md emits a :root { ... } block of CSS custom properties derived from the token set. This makes DESIGN.md tokens directly consumable in native-CSS codebases without Tailwind or any token tooling.

Why this matters

Issue #15 asks for a CSS variables export so DESIGN.md is useful outside the Tailwind ecosystem and gives coding agents a direct artifact to apply in native CSS. It complements the existing dtcg and Tailwind exports. Scope here is the core format plus --prefix; dark-theme / media-query variants are deferred as the issue notes.

Changes

• New emitter under packages/cli/src/linter/css-vars/ (spec.ts, handler.ts, serialize.ts) following the DtcgEmitterHandler pattern: a pure execute(state) maps colors, spacing, and rounded to --<group>-<name> declarations, and serializeCssVars renders the :root block.
• Wired into export.ts: css-vars added to the FORMATS enum, a new branch mirroring the other formats, and an optional --prefix arg threaded into the serializer (default emits --color-...).
• Re-exported the handler, serializer, and result type from linter/index.ts.
• Nested token keys flatten to dotted names (e.g. background.light); those dots are collapsed to hyphens so the emitted property name is valid CSS.

Sample output (--prefix ds):

:root {
  --ds-color-surface: #f9f9ff;
  --ds-spacing-unit: 8px;
  --ds-rounded-sm: 0.25rem;
}

Testing

• bun test in packages/cli: 289 pass, 1 skip, 0 fail
• bun run lint (tsc --noEmit): clean
• New unit tests cover colors, spacing/rounded units, --prefix, nested-name hyphenation, and empty input; a command-level test covers a --format css-vars round-trip plus the invalid-format exit-code-1 path.

Fixes #15