Add commit message normalizer hook and install script

- scripts/normalize-commit-msg.js: capitalizes subject, strips trailing
  period, trims whitespace, warns when subject > 72 chars
- scripts/commit-msg: shell wrapper symlinked into .git/hooks/commit-msg
- scripts/install-hooks.sh: contributor setup script (sh scripts/install-hooks.sh)
- scripts/package.json: test runner + hooks:install npm script
- scripts/__tests__/normalize-commit-msg.test.js: 15 unit tests

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

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

@@ -0,0 +1,90 @@
+import { describe, it, expect } from 'vitest';
+const { normalizeSubject, normalizeMessage } = require('../normalize-commit-msg.js');
+
+describe('normalizeSubject', () => {
+  it('passes an already-valid subject unchanged', () => {
+    const { subject, warned } = normalizeSubject('Add feature flag support');
+    expect(subject).toBe('Add feature flag support');
+    expect(warned).toBe(false);
+  });
+
+  it('capitalizes the first letter', () => {
+    const { subject } = normalizeSubject('add feature flag support');
+    expect(subject).toBe('Add feature flag support');
+  });
+
+  it('strips a trailing period', () => {
+    const { subject } = normalizeSubject('Add feature flag support.');
+    expect(subject).toBe('Add feature flag support');
+  });
+
+  it('trims leading whitespace', () => {
+    const { subject } = normalizeSubject('  Fix the bug');
+    expect(subject).toBe('Fix the bug');
+  });
+
+  it('trims trailing whitespace', () => {
+    const { subject } = normalizeSubject('Fix the bug   ');
+    expect(subject).toBe('Fix the bug');
+  });
+
+  it('capitalizes and strips period together', () => {
+    const { subject } = normalizeSubject('fix the bug.');
+    expect(subject).toBe('Fix the bug');
+  });
+
+  it('does not strip a period that is not trailing', () => {
+    const { subject } = normalizeSubject('Fix bug in v1.0 release');
+    expect(subject).toBe('Fix bug in v1.0 release');
+  });
+
+  it('warns when subject exceeds 72 characters', () => {
+    const long = 'A'.repeat(73);
+    const { warned } = normalizeSubject(long);
+    expect(warned).toBe(true);
+  });
+
+  it('does not warn when subject is exactly 72 characters', () => {
+    const exact = 'A'.repeat(72);
+    const { warned } = normalizeSubject(exact);
+    expect(warned).toBe(false);
+  });
+
+  it('does not warn when subject is under 72 characters', () => {
+    const { warned } = normalizeSubject('Short message');
+    expect(warned).toBe(false);
+  });
+
+  it('handles an empty subject gracefully', () => {
+    const { subject, warned } = normalizeSubject('');
+    expect(subject).toBe('');
+    expect(warned).toBe(false);
+  });
+});
+
+describe('normalizeMessage', () => {
+  it('normalizes only the subject line of a multi-line message', () => {
+    const input = 'fix the bug.\n\nThis is the body paragraph.';
+    const { message } = normalizeMessage(input);
+    expect(message).toBe('Fix the bug\n\nThis is the body paragraph.');
+  });
+
+  it('skips comment lines when finding the subject', () => {
+    const input = '# Comment\nfix the bug.';
+    const { message } = normalizeMessage(input);
+    expect(message).toBe('# Comment\nFix the bug');
+  });
+
+  it('returns warned true for long subject inside full message', () => {
+    const longSubject = 'x'.repeat(73);
+    const input = `${longSubject}\n\nBody.`;
+    const { warned } = normalizeMessage(input);
+    expect(warned).toBe(true);
+  });
+
+  it('preserves body lines exactly as-is', () => {
+    const input = 'Fix bug\n\n  - detail one\n  - detail two.';
+    const { message } = normalizeMessage(input);
+    expect(message).toBe('Fix bug\n\n  - detail one\n  - detail two.');
+  });
+});