Identities live in ~/.git-identities (alias|Name|email). Interactive
menu to select, add, or remove identities; direct mode via alias
argument; --global/-g scope flag (defaults to local inside a repo,
global fallback outside); --list/--current helpers. Full --dryrun
support and a 27-assertion test suite driven through dryrun.
GIT_IDENTITIES_FILE env override for testability.
