Using Puck as a single button keyboard

Take a look at http://www.espruino.com/Quick+Start#start-writing-code- - or in fact a lot of the other tutorials mention it as well. You just type save() on the left hand side of the IDE when your code is working, and it will be saved to flash.

You might also want to execute something when power is applied - for instance the setServices command - so just do that inside an onInit() command.

And no - you can't emulate a keyboard over Wifi, so you'd have to do something completely different for the Espruino Wifi, writing some software for your PC that would communicate with Espruino over the network connection