Fix test import style and document hooks in CLAUDE.md

- Replace mixed ESM/CJS import syntax in test file with consistent ESM
  imports throughout (both vitest and normalizer use import)
- Add Git Hooks section to CLAUDE.md documenting install-hooks.sh
  and the commit-msg normalizer for new contributors

Nightshift-Task: commit-normalize
Nightshift-Ref: https://github.com/marcus/nightshift

scripts/__tests__/normalize-commit-msg.test.js

@@ -1,5 +1,5 @@
 import { describe, it, expect } from 'vitest';
-const { normalizeSubject, normalizeMessage } = require('../normalize-commit-msg.js');
+import { normalizeSubject, normalizeMessage } from '../normalize-commit-msg.js';
 
 describe('normalizeSubject', () => {
   it('passes an already-valid subject unchanged', () => {