Add color system rules to design standards

Established core color constants:
- Pure black: #0a0a0a (never #000000)
- White text: #FAFAFA (never #FFFFFF)
- Reverse in light mode (bg: #FAFAFA, text: #0a0a0a)

Updated:
- visual-design-system.mdc: Added core color constants section
- visual-design-system.mdc: Updated dark mode CSS variables
- code-quality.mdc: Added color consistency checklist

These rules ensure consistent color usage across the entire application.

Co-authored-by: Cursor <cursoragent@cursor.com>

.cursor/rules/code-quality.mdc

@@ -162,6 +162,12 @@ import './styles.css'
 
 ## Code Review Checklist
 
+### Color Consistency
+- ✅ Pure black is `#0a0a0a` (not `#000000`)
+- ✅ White text is `#FAFAFA` (not `#FFFFFF`)
+- ✅ Colors use CSS variables or design tokens
+- ✅ Light/dark mode properly implemented
+
 ### Before Submitting PR
 - ✅ Code runs without errors
 - ✅ Tests pass