Public entrypoint: install with auto-detected style. Preserves the
previous signature so existing callers (setup.rs, cli/shell_init.rs)
don’t need to change.
The source <rc> command for the user’s current login shell ($SHELL), or
None when it cannot be determined. Single source of truth so post-setup
and post-update hints stay in sync and never advise sourcing a shell the
user does not have (e.g. ~/.zshrc on a bash-only system — see #321).