feat: add design tokens as CSS custom properties

Create src/styles/tokens.css with complete design token system including:
- Surface, text, accent, code, border, focus colors
- State colors (success, warning, error, info)
- Shadow definitions
- Typography families with full fallback stacks
- Typography scale (xs through 3xl) and line heights
- Spacing scale (0 through 12)
- Border radius tokens
- Layout constraints (prose, code, container widths)
- Motion tokens with reduced-motion media query

All values sourced from .design/specs/design-tokens-2026-05-26.md.
Includes verification test with 56 assertions across all categories.

src/styles/tokens.css

@@ -0,0 +1,120 @@
+:root {
+  /* Surfaces */
+  --surface-base: #F5F0E8;
+  --surface-card: #FEFCF7;
+  --surface-code: #EEEAE2;
+  --surface-inset: #E8E3DA;
+
+  /* Text */
+  --text-primary: #2B2421;
+  --text-secondary: #6B5F55;
+  --text-tertiary: #948880;
+
+  /* Accents */
+  --accent-primary: #A0522D;
+  --accent-primary-hover: #8B4726;
+  --accent-secondary: #7D6C2F;
+  --accent-secondary-hover: #6B5C28;
+
+  /* Code */
+  --code-text: #3D342E;
+
+  /* Borders */
+  --border-subtle: #DDD7CD;
+  --border-default: #C2BAA9;
+  --border-strong: #8E857A;
+
+  /* Focus */
+  --focus-ring: #A0522D;
+
+  /* State: Success */
+  --success-bg: #E8EDDF;
+  --success-text: #4A5E3A;
+  --success-border: #B5C4A0;
+
+  /* State: Warning */
+  --warning-bg: #F2E8D0;
+  --warning-text: #7A5C1F;
+  --warning-border: #D4BC7C;
+
+  /* State: Error */
+  --error-bg: #F2DED6;
+  --error-text: #8B3A2A;
+  --error-border: #CFA090;
+
+  /* State: Info */
+  --info-bg: #E5E2DA;
+  --info-text: #4A4540;
+  --info-border: #B8B2A8;
+
+  /* Shadows */
+  --shadow-sm: 0 1px 3px rgba(43, 36, 33, 0.04);
+  --shadow-md: 0 2px 8px rgba(43, 36, 33, 0.06);
+
+  /* Typography families */
+  --font-prose: 'Source Serif 4', 'Source Serif Pro', Georgia, 'Times New Roman', serif;
+  --font-structure: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
+  --font-evidence: 'JetBrains Mono', 'Fira Code', 'SF Mono', Consolas, monospace;
+
+  /* Typography scale */
+  --type-xs: 0.75rem;
+  --type-sm: 0.875rem;
+  --type-base: 1.125rem;
+  --type-lg: 1.375rem;
+  --type-xl: 1.75rem;
+  --type-2xl: 2.25rem;
+  --type-3xl: 2.75rem;
+
+  /* Line heights */
+  --leading-tight: 1.15;
+  --leading-snug: 1.25;
+  --leading-heading: 1.35;
+  --leading-normal: 1.5;
+  --leading-relaxed: 1.6;
+  --leading-prose: 1.7;
+
+  /* Spacing */
+  --space-0: 0;
+  --space-px: 1px;
+  --space-0-5: 0.25rem;
+  --space-1: 0.5rem;
+  --space-1-5: 0.75rem;
+  --space-2: 1rem;
+  --space-2-5: 1.25rem;
+  --space-3: 1.5rem;
+  --space-4: 2rem;
+  --space-5: 2.5rem;
+  --space-6: 3rem;
+  --space-8: 4rem;
+  --space-10: 5rem;
+  --space-12: 6rem;
+
+  /* Border radius */
+  --rounded-none: 0;
+  --rounded-sm: 2px;
+  --rounded-md: 4px;
+  --rounded-lg: 6px;
+  --rounded-xl: 8px;
+  --rounded-full: 9999px;
+
+  /* Layout */
+  --width-prose: 45rem;
+  --width-code: 52.5rem;
+  --width-container: 68.75rem;
+  --gutter: 1.5rem;
+
+  /* Motion */
+  --duration-fast: 100ms;
+  --duration-normal: 150ms;
+  --duration-slow: 250ms;
+  --ease-default: cubic-bezier(0.25, 0.1, 0.25, 1.0);
+  --ease-in-out: cubic-bezier(0.45, 0.05, 0.55, 0.95);
+}
+
+@media (prefers-reduced-motion: reduce) {
+  :root {
+    --duration-fast: 0ms;
+    --duration-normal: 0ms;
+    --duration-slow: 0ms;
+  }
+}