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/install-hooks.sh

@@ -0,0 +1,34 @@
+#!/bin/sh
+# install-hooks.sh
+# Installs the project's git hooks into .git/hooks/.
+# Run this once after cloning: sh scripts/install-hooks.sh
+
+set -e
+
+REPO_ROOT="$(git rev-parse --show-toplevel)"
+HOOKS_DIR="$REPO_ROOT/.git/hooks"
+SCRIPTS_DIR="$REPO_ROOT/scripts"
+
+install_hook() {
+  local name="$1"
+  local src="$SCRIPTS_DIR/$name"
+  local dst="$HOOKS_DIR/$name"
+
+  if [ ! -f "$src" ]; then
+    echo "install-hooks: source not found: $src" >&2
+    return 1
+  fi
+
+  if [ -f "$dst" ] && [ ! -L "$dst" ]; then
+    echo "install-hooks: warning: $dst already exists and is not a symlink — skipping"
+    return 0
+  fi
+
+  ln -sf "$src" "$dst"
+  chmod +x "$src"
+  echo "install-hooks: installed $name -> $dst"
+}
+
+install_hook "commit-msg"
+
+echo "install-hooks: done"