I keep being a little puzzled over the frequent use of the "agenty" term at LW, since I haven't really seen any arguments establishing why this would be a useful distinction to make in the first place.

Here is my brief impression of what the term "agenty" on LW means:

An "agent" is a person with surplus executive function.

"Executive function" is some combination of planning ability, willpower, and energy (only somewhat related to the concept in psychology). "Surplus" generally means "available to the labeler on the margin." Supposing that people have some relatively fixed replenishing supply of executive function, and relatively fixed consistent drains on executive function, then someone who has surplus executive function today will probably have surplus executive function tomorrow, or next week, or so on. They are likely to be continually starting and finishing side projects.*

The practical usefulness of this term seems obvious: this is someone you can delegate to with mission-type tactics (possibly better known as Auftragstaktik). This ability makes them good people to be friends with. Having this ability yourself both makes you better able to achieve your goals and makes you a more valuable friend, raising the quality of the people you can be friends with.

*Someone who has lots of executive function, but whose regular demands take all of it, will have lots of progress in their primary work but little progress on side projects. Someone whose demands outstrip their supply is likely to be dropping balls and burning out.