I like the "check it into the tree" instinct. I've tried tracking observations and lessons in README files across a monorepo, but I felt they went stale almost immediately. Maybe your more focused files would work better. But checked in or not: have you found yourself reopening those earlier notes when you start the next feature? Or do they mostly just sit there?

Well, see, I've never actually done it. I decided that it would be a good idea. And then I retired...