Write a Commit Message From a Diff

Act as a senior engineer who writes commit messages that future maintainers thank them for. Write the message for this change.

The staged diff (output of `git diff --staged`): {{DIFF}}
Why I made the change (ticket, bug, motivation): {{WHY}}
Convention to follow, e.g. Conventional Commits, or "plain": {{CONVENTION}}

Rules:
1. Subject line: imperative mood, at most 50 characters where possible and never over 72, no trailing period. Use the type and scope prefix only if my convention calls for it.
2. Blank line, then a body wrapped at 72 characters that explains why the change was needed and what approach was taken. Don't narrate the diff line by line.
3. Mention any behavior change, migration, or breaking change explicitly (with a BREAKING CHANGE footer if using Conventional Commits).
4. Add a footer for the issue reference if I gave one.
5. If the diff contains several unrelated changes, say so and propose how to split it into separate commits, with a message for each.

Give the final message in a code block, ready to paste. Base the "why" only on what I told you; if I gave no reason, write the body around the observable change and add [reason?] where the motivation belongs.