> But while it would make a great recording of project history, I wonder if a lot of it doesn't end up detailing blind alleys the project went down and had to back out of

Yes, I've run into this as well. I had the agent document the changes it would make along the way, but it would want to keep making note of things that we switched away from. Eg, "In phase3 we used mysql instead of sqlite". On some projects I let the agwnt do all the documentation and plan.md and state.md files, and every so often go through manually and delete some of the cruft that's no longer needed.