Implements a cooldown period to prevent too-frequent npm package publishes:

- Adds cooldown check before publishing (24-hour minimum between publishes)
- Fails gracefully with clear message showing time remaining if cooldown active
- Updates timestamp after successful publish using GitHub repository variables
- Uses LAST_NPM_PUBLISH_TIME variable to track last publish time

The workflow will now:
1. Check if 24 hours have passed since last publish
2. Display time remaining if cooldown still active
3. Proceed with normal publish if cooldown expired
4. Update timestamp after successful publish

The actions/setup-node action already handles npm authentication
automatically when registry-url is configured, so the explicit
NODE_AUTH_TOKEN environment variable is redundant.