Assisted-by: Claude Sonnet 4.5 via Crush

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