Add Zenodo DOI badge to README

Published workflows and test data to Zenodo as v1.0.0:
https://doi.org/10.5281/zenodo.19233880

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: inutano

README.md

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 # pa-cwl: Pretty Agentic CWL
 
+[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.19233880.svg)](https://doi.org/10.5281/zenodo.19233880)
+
 Production-ready [Common Workflow Language](https://www.commonwl.org/) workflows designed for AI-agent execution via the [GA4GH Workflow Execution Service (WES)](https://ga4gh.github.io/workflow-execution-service-schemas/) API.
 
 ## What is this?