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/normalize-commit-msg.js

@@ -0,0 +1,95 @@
+#!/usr/bin/env node
+/**
+ * normalize-commit-msg.js
+ *
+ * Git commit-msg hook: reads the commit message file, applies normalization
+ * rules to the subject line, rewrites the file in place.
+ *
+ * Rules:
+ *   1. Trim leading/trailing whitespace from the subject line
+ *   2. Capitalize the first letter of the subject
+ *   3. Strip a trailing period from the subject
+ *   4. Warn (but do not block) if the subject exceeds 72 characters
+ */
+
+'use strict';
+
+const fs = require('fs');
+
+const MAX_SUBJECT_LEN = 72;
+
+/**
+ * Normalize the subject line of a commit message.
+ * Returns { subject, warned } where warned is true if a length warning was emitted.
+ *
+ * @param {string} subject
+ * @returns {{ subject: string, warned: boolean }}
+ */
+function normalizeSubject(subject) {
+  let s = subject.trimEnd();
+
+  // Trim leading whitespace
+  s = s.trimStart();
+
+  // Capitalize first letter
+  if (s.length > 0) {
+    s = s[0].toUpperCase() + s.slice(1);
+  }
+
+  // Strip trailing period
+  if (s.endsWith('.')) {
+    s = s.slice(0, -1);
+  }
+
+  const warned = s.length > MAX_SUBJECT_LEN;
+
+  return { subject: s, warned };
+}
+
+/**
+ * Normalize a full commit message string.
+ * Only the subject line (first non-empty, non-comment line) is modified.
+ *
+ * @param {string} message
+ * @returns {{ message: string, warned: boolean }}
+ */
+function normalizeMessage(message) {
+  const lines = message.split('\n');
+  let warned = false;
+
+  // Find the subject line (first non-comment line)
+  for (let i = 0; i < lines.length; i++) {
+    const line = lines[i];
+    if (!line.startsWith('#')) {
+      const result = normalizeSubject(line);
+      lines[i] = result.subject;
+      warned = result.warned;
+      break;
+    }
+  }
+
+  return { message: lines.join('\n'), warned };
+}
+
+// Only run as a hook when invoked directly (not when required in tests)
+if (require.main === module) {
+  const msgFile = process.argv[2];
+  if (!msgFile) {
+    process.stderr.write('commit-msg hook: no message file argument\n');
+    process.exit(1);
+  }
+
+  const original = fs.readFileSync(msgFile, 'utf8');
+  const { message, warned } = normalizeMessage(original);
+
+  if (warned) {
+    process.stderr.write(
+      `commit-msg warning: subject line exceeds ${MAX_SUBJECT_LEN} characters — consider shortening it.\n`
+    );
+  }
+
+  fs.writeFileSync(msgFile, message, 'utf8');
+  process.exit(0);
+}
+
+module.exports = { normalizeSubject, normalizeMessage };