If my boss tells me to fix a bug in our database, and I fix it, is it really my boss who fixed it, since he has the agency and I'm just a tool he uses?

I'd say that people's motivation and agency, like their taste, creativity, and opinions, are shaped by external inputs, so saying that their agency is "owned" by them is a stretch.

The alternative would be the proposition that a slave, lacking agency, has no ability to solve problems. I find that idea unpalatable.