Extends the tool to support both commit and tag formatting:

- Add git-formatted-tag mode for annotated tag formatting
- Add install subcommand to create symlinks in ~/.local/bin
- Extract shared code (runGitWithStdin, validateSubjectLength) to git.go
- Update module path to git.secluded.site/git-format

BREAKING CHANGE: Renamed from formatted-commit to git-format. Uninstall
the old binary and reinstall following README instructions. Run
git-format install to create symlinks, then invoke via git
formatted-commit or git formatted-tag.

The -B flag now accepts a string argument containing the breaking change
description instead of being a boolean flag. This description is
formatted and inserted as a BREAKING CHANGE: footer between the body and
git trailers, following the Conventional Commits specification.

The flag still adds ! to the subject line. When used with heredoc
syntax, multi-line breaking change descriptions are supported.

Also re-added the -a/--amend flag that was present in main but missing
from this feature branch.

Implements: bug-e75a648
Assisted-by: Claude Sonnet 4.5 via Crush