I think Makefile is maybe the wrong analogy - the problem with most people and makefiles is they write so few of them, the general idea of what make does is at hand, but the muscle memory of how to do it from scratch is not. But, point taken - I've seen so much code copy-pasta'd from the web, there will be like a bunch of dead stuff in it that's actually not used. A good practice here is to keep deleting stuff until…
The repository is personal, and contains info on tools that are publicly available.
I keep organisation specific knowledge in a similar but separate repo, which I discard when my tenure with a client or employer ends.