schacon/blink ? reverse-engineered prompt

Build me a tiny Arduino sketch that blinks the built in LED on and off in a loop.

I want it to be as simple as possible, with clear code comments so someone new to Arduino can understand what each part does. Make sure it works on a standard board using the usual LED pin, and include the basic setup and loop structure.

If there’s anything subtle about different Arduino boards, note that briefly in the code comments or a short README note.