Fold follow-up fixes into earlier commits with git commit --fixup and git rebase --autosquash

Documented steps

1. While working, instead of letting fix-up changes become loose commits, create a marker commit that targets an existing commit: `git commit --fixup=<target>` stages current content under a 'fixup!' subject that `--autosquash` understands.
2. To also replace the targeted commit's message, use `git commit --fixup=amend:<target>` (which creates an 'amend!' commit whose message you refine in the editor), or `--fixup=reword:<target> --only` to change only the message.
3. Alternatively use `git commit --squash=<target>` to create a 'squash!' commit whose message you want combined with the target via `-m`/`-c`/`-C`/`-F`.
4. When the target commits are local (not yet pushed), run `git rebase -i --autosquash <base>` — Git auto-arranges the fixup!/squash! commits directly beneath their targets with the correct `fixup`/`squash` verbs already applied.
5. This folds each fixup into the intended commit with the combined message, keeping history clean without manual reordering or message editing.
6. Verify with `git log --oneline` that the fixup commits collapsed into their targets.
7. Official docs: https://git-scm.com/docs/git-commit