The Cody Computer doesn't use a PS/2 keyboard, it's actually using the 65C22 VIA and a CD4051 to scan the keyboard matrix. This is mentioned in the book and in the schematic up on Github, but for a quick version, it's conceptually very similar to the Oric (as mentioned on Garth Wilson's site: https://wilsonminesco.com/6502primer/potpourri.html).

But for the Propeller itself, there's actually a driver for PS/2 keyboards and mice written by Kwabena Agyeman. It's up on the OBEX (https://obex.parallax.com/obex/combo-ps2-keyboard-and-mouse-...) which is a good place to look for Propeller-related code. I've never used that driver personally, but that's where I would start.