Add CI status badge to README (#2)

Author: HelloThisWorld

README.md

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 # agent-skill-forge
 
+[![CI](https://github.com/HelloThisWorld/agent-skill-forge/actions/workflows/ci.yml/badge.svg?branch=main)](https://github.com/HelloThisWorld/agent-skill-forge/actions/workflows/ci.yml)
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 A spec-driven AI agent skill generator and verification pipeline for building, testing, repairing, and packaging production-ready agent skills.
 
 Most AI agent skills are still created as prompt files and manually inspected. `agent-skill-forge` turns skill creation into an engineering pipeline: