docs: Add Zenodo DOI 10.5281/zenodo.18048903

Updated citation files with official Zenodo DOI:
- CITATION.cff: Added doi field
- README.md: Updated DOI badge

PACE Pattern is now officially citable in academic work.

Author: michshat

CITATION.cff

@@ -17,6 +17,7 @@ date-released: 2025-12-24
 license: CC-BY-4.0
 repository-code: "https://github.com/semanticintent/pace-pattern"
 url: "https://semanticintent.dev"
+doi: "10.5281/zenodo.18048903"
 keywords:
   - agentic-ux
   - conversational-commerce

README.md

@@ -1,6 +1,6 @@
 # PACE: Pattern for Agentic Conversational Experience
 
-[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.XXXXXXX.svg)](https://doi.org/10.5281/zenodo.XXXXXXX)
+[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.18048903.svg)](https://doi.org/10.5281/zenodo.18048903)
 [![License: CC BY 4.0](https://img.shields.io/badge/License-CC%20BY%204.0-lightgrey.svg)](https://creativecommons.org/licenses/by/4.0/)
 [![Reference Implementation](https://img.shields.io/badge/reference-MillPond-blue)](https://millpond-storefront.pages.dev)