That's easy enough: the hotel management for allowing their staff to blame agoda when, by their own admission, they know that they "always makes mistakes". The hotel, by insisting it was all agoda's fault, basically insisted that you were an idiot for using them.
Agoda has no way to add a free breakfast to a hotel's offer, either mistakenly or dishonestly. They display the room rates and offers given them by the hotel. Maybe the hotel did offer included breakfast previously, changed that offer and either haven't updated their agoda listing or agoda has missed the change. Nevertheless, the hotel hasn't done anything about it, knows there's an issue but preferring the "always makes mistakes" excuse while taking advantage of customers being wrongly advised.