Complete the Git commands: Git mktag: Construct Tag Objects with Validation

In a disposable repository, create one commit and record its object ID. Construct a valid annotated-tag object input containing that commit as the target, a tag name, tagger identity, timestamp, and message. Run `git mktag` and capture the printed object ID. Verify with `git cat-file -t` that it is a tag object and with `git cat-file -p` that the stored contents match your input. Then change only the annotation message, run `git mktag` again, and demonstrate that the object ID changes. Finally, deliberately use a nonexistent target object and show that `git mktag` rejects the input. Do not create a tag reference with `git tag`; the exercise is specifically about the tag-object layer.