Update CLAUDE.md with build commands and project conventions

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -32,6 +32,11 @@ These are the non-negotiable principles of this project. Every decision — arch
 - For every feature, critically evaluate what data is necessary. Only store what is absolutely required for functionality.
 - Never include external dependencies that phone home: no CDNs, no Google Fonts, no tracking-capable libraries.
 
+### Design
+
+- The visual design system is defined in `spec/design-system.md`. All frontend implementation must follow it.
+- Color palette, typography, component patterns, and layout rules are specified there — do not deviate without explicit approval.
+
 ### Quality
 
 - KISS and grugbrain. Engineer it properly, but don't over-engineer.