What does "an action that harms another agent" mean? For instance, if I threaten to not give you a chicken unless you give me $5, does "I don't give you a chicken" count as "a course of action that harms another agent"? Or does it have to be an active course, rather than act of omission?

It's not blackmail unless, given that I don't give you $5, you would be worse of, CDT-wise, not giving me the chicken than giving me the chicken. Which is to say, you really want to give me the chicken but you're threatening to withhold it because you think you can make $5 out of it. If I were a Don't-give-$5-bot, or just broke, you would have no reason to threaten to withhold the chicken. If you don't want to give me the chicken, but are willing to do so if I give you $5, that's just normal trade.