create-sibling-gin offer known unavailable sibling name

[mih]

The default is gin. It remains the default, even when a gin sibling already exists. It should either offer a different name, or no name, rather than suggesting all is good and then fail eventually.

At least when existing=error

[adswa]

I think this is true for all commands with default sibling names. create-sibling-github is also affected. create-sibling-webdav isn't; it doesn't set a default sibling name in its signature.

I'm wondering what a good alternative may be. create-sibling-webdav will, by default, use the hostname of the WebDAV url (which might also be already taken). If we find a way to update the defaults in gooey for gin and github, maybe simply appending numbers (gin-1, gin-2, ...) might work?