Comment on any answer, assign it to someone, and resolve it — decisions stay next to the reasoning.
Any part of a thread can be commented on. A comment can be assigned to a person, which turns it into an action item with an owner, and resolved when it is done.
The point is that the follow-up lives next to the reasoning that produced it. The usual alternative is a Slack thread that has lost all its context by the second reply.
Assignments notify the person they belong to, and the open ones stay visible so nothing quietly falls off the end of a long session.