Built-in skill — conventional commit messages from diffs.
Injects a system-prompt section specifying the type(scope): description
conventional-commit format, an imperative subject of at most 72 characters, a
body that explains why rather than what, and BREAKING CHANGE: footers.
Declares no keywords, so it never self-activates: name it explicitly with
skills: [Nous.Skills.CommitMessage] or pull in skills: [{:group, :git}].
Group :git; tags :git, :commit, :vcs.