docs: add CITATION.cff

[Harperbot]

Summary

• Add CITATION.cff (Citation File Format 1.2.0) so the repo is properly citable.
• Enables GitHub's "Cite this repository" button + structured APA/BibTeX export.
• Metadata: title / abstract / author (Harper Systems) / version 1.1.0 / MIT / repo URL / keywords.

Note

• No DOI yet — a Zenodo DOI requires a Zenodo account (OAuth + repo enable + release archive). Deferred per "no account registration" constraint. CITATION.cff alone still lets people cite by repo URL + version.
• Pure doc addition: no code / dependency / behavior change.

Test plan

• python3 -c "import yaml; yaml.safe_load(open('CITATION.cff'))" parses clean
• GitHub renders "Cite this repository" button on the repo home after merge