The code in the plugin seen at: Cardo — A TiddlyWiki for Getting Stuff Done Click to Get Cardo says:
For me that says the contribution is bound to the TW CLA (Contributor License Agreement). Which IMO is a clear intent, that all the terms used with the TW CLA are valid for the Cardo project too.
This basically means: The code is BSD and documentation is CC-BY. So we should be good to use and modify it under the same terms.
Edit the last known link to cardo.wiki at archive.org also shows the same license info.