[codex] Add CITATION.cff

[gvegayon]

GitHub cannot presently discover structured citation metadata for ABCoptim. This pull request adds a root CITATION.cff, excludes it from the R source tarball, adds a read-only schema-validation workflow, and records the metadata addition in NEWS.

The existing inst/CITATION remains the source for citation("ABCoptim"); the CFF is repository metadata for GitHub and other CFF consumers.

Validated with cffconvert --validate, cffr::cff_validate(), and R CMD build --no-build-vignettes; the source tarball excludes CITATION.cff.

Note: This pull request was prepared by GPT-5.6 Terra on behalf of gvegayon.

[github-actions]

Version bump check ✅

group	base	head	bump
package	0.15.0-99	0.15.0-99	⏭️ skipped

_{Checked 1 group(s) against 9ebed30896b98640a7ed3797a82403c08f4269b3 · please-bump}

[github-actions]

Thank you for your contribution @gvegayon 🚀! The pkgdown site preview is ready for review 👉 Download here 👈!
_{(The artifact expires on 2026-12-10T05:22:59Z. You can re-generate it by re-running the workflow here.)}