I want to mod the way the AI behaves: changing the way certain orders are implemented, adding new orders etc.
I found data/behaviours.csv, but that is only a listing of the various orders and how they should be presented to the player; not how they are actually implemented.
Does anyone know how/where to do this?
Short answer: u cant do that.
You got it. 
Neighbour, the behavior you describe is a part of the game’s core codebase. It’s therefore well beyond the access of any modder. Sorry, mate.
Check out this excellent thread for major insight on how to squeeze as much performance as possible out of the game’s AI. That discussion should be required reading for all players.
Awww…and there’s no possibility for cliffsky to make this available to other programmers? 
I wouldn’t mind rummaging around to make the ai pluggable/configurable.