docs(citation): add software citation metadata

[emmepra]

Summary

• add CITATION.cff so GitHub exposes a repository citation entry
• add .zenodo.json so Zenodo archives releases with cleaner software metadata
• document the citation and first-release DOI flow in the README

Scope

• in scope:
  • software citation metadata
  • Zenodo release metadata
  • maintainer/user citation guidance in the README
• out of scope:
  • DOI badge insertion before the first Zenodo DOI exists
  • software-paper publication workflow

Validation

• relevant tests/checks were run
• Docker/runtime validation was run when the change touched backend, orchestration, templates, or integration paths
• docs/rules were updated when behavior, contracts, or workflow changed

Commands run:

ruby -e "require 'yaml'; YAML.load_file('CITATION.cff'); puts 'CITATION.cff OK'"
python3 -c "import json; json.load(open('.zenodo.json')); print('.zenodo.json OK')"

Risks

• user-visible regressions: low; docs/metadata only
• operational/runtime risks: none expected
• follow-up work:
  • after the first Zenodo-backed release, add the minted DOI badge and DOI citation example to the README

Evidence

• logs: local YAML/JSON parse validation
• screenshots: none
• related issues: none