Add TUI screenshot to README

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: cpetersen

README.md

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 A complete Ruby implementation of Retrieval-Augmented Generation (RAG) pipeline using native Ruby ML/NLP gems.
 
+<p align="center">
+  <img src="/docs/assets/screenshot.png" alt="ragnar TUI" width="600">
+</p>
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 ## Overview
 
 Ragnar provides a production-ready RAG pipeline for Ruby applications, integrating: