fix: convert design-diff-analyzer to ES modules

- Use import instead of require (package.json has type: module)
- Fixes script execution for post-commit hook

scripts/design-diff-analyzer.js

@@ -4,8 +4,8 @@
  * Analyzes changes to ui-design.pen and generates implementation tasks for Claude Code
  */
 
-const { execSync } = require('child_process');
-const fs = require('fs');
+import { execSync } from 'child_process';
+import fs from 'fs';
 
 function getDiff() {
   try {