> So you made a commit. What artifacts change as a result? What do you need to rebuild, retest, and redeploy?

If you're using Git, then typically for each push to the remote repository you get a notification with this data in it:

  BRANCH        # the remote branch getting updated
  OLD_COMMIT    # the commit the branch ref was pointing to before the push
  NEW_COMMIT    # the commit the branch ref was pointing to after the push

  # To get the list of files that changed in the push:
  git diff --name-only "$OLD_COMMIT" "$NEW_COMMIT"

Once you know which files changed in a push you can figure out which artifacts you need to build. Right now you'll have to write that tooling yourself since I don't know of any off-the-shelf tools that do it. In my company's case, we have "project.yml" files scattered through the repo telling us which directories have buildable artifacts and what branches each one needs to be built for. The tooling to support this is a few hundred lines of Bash and Python. In our case we're still small enough that we can brute force some stuff, but we can easily improve the tooling as we go along.