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/commit-msg

@@ -0,0 +1,4 @@
+#!/bin/sh
+# Git commit-msg hook — delegates to normalize-commit-msg.js
+# This file is symlinked into .git/hooks/commit-msg by scripts/install-hooks.sh
+node "$(git rev-parse --show-toplevel)/scripts/normalize-commit-msg.js" "$1"