mkrep can now pick a git host from --server, $GIT_SERVER, and
$GITEA_URL/$GITEA_HOST/$GITLAB_URL/$GITLAB_HOST (base URL only -- type
must come from --server/$GIT_SERVER, so setting those URL vars for an
unrelated tool can't silently turn a plain mkrep call into a
remote-creating one). Before creating anything it checks via
gh/glab/tea whether the repo already exists and links instead of
recreating it; --check-existing runs just that check and reports
without touching remotes.
