clabtoolkit 0.5.1

clabtoolkit 0.5.1 adds a CITATION.cff file with a reference to the toolkit paper (arXiv:2607.02638), so GitHub now offers "Cite this repository" and includes it in the source distribution. It also updates the release instructions in CONTRIBUTING.rst (version and date in CITATION.cff, and the GitHub release that archives each version in Zenodo). There are no changes to the library code with respect to 0.5.0, so it is a drop-in replacement. Install it with pip install clabtoolkit==0.5.1. The full list of changes is in HISTORY.rst.