Rename to git-format and add tag mode

⚠️ BREAKING CHANGES
- Renamed from formatted-commit to git-format; reinstall per README
- New invocation: use 'git formatted-commit' and 'git formatted-tag' via
  git

✨ Features
- [tag]: add formatted-tag mode for annotated git tags
- [install]: add install subcommand to create symlinks in ~/.local/bin

♻️ Code Refactoring
- [git]: extract shared runGitWithStdin and validateSubjectLength
- Update module path to git.secluded.site/git-format
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