It says I need "robes" skill to wear the robes. There is no such skill on my character screen. Is this a bug?
I played through all the way without noticing this, since my first time I used armor. Just trying something different this time around.

I think it is a bug, yes. If you look at the screenshots on the game page here, you will see one which has all the skills including robes.

I have it also, no way to skill up or equip robes at least on my version / pc configuration.
Hope it gets fixed soon, the developer seems a nice guy!

So does nobody have any clue of a workaround? a hidden INI setting to change or anything? maybe an unofficial patch out there?

Workaround: Download the game installer from the creator instead of through GoG until the GoG version is fixed. Use the "gog..." key you got after buying this game. It's DRM free, with the exception of challenges which require you to be logged in if you want the rewards.

I'm sure that if we ask the developer nicely enough, we can show him why he should bring those rewards to the GoG version as well, perhaps through another in-game store (for in-game credits).