Add JOSS paper status badge to README

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

Author: brycewang-stanford

README.md

@@ -5,6 +5,7 @@
 [![License: MIT](https://img.shields.io/badge/License-MIT-blue.svg)](https://github.com/brycewang-stanford/statspai/blob/main/LICENSE)
 [![Tests](https://github.com/brycewang-stanford/statspai/workflows/CI%2FCD%20Pipeline/badge.svg)](https://github.com/brycewang-stanford/statspai/actions)
 [![PyPI Downloads](https://static.pepy.tech/personalized-badge/statspai?period=total&units=INTERNATIONAL_SYSTEM&left_color=BLACK&right_color=GREEN&left_text=downloads)](https://pepy.tech/projects/statspai)
+[![status](https://joss.theoj.org/papers/9f1c837b1b1df7adfcdd538c3698e332/status.svg)](https://joss.theoj.org/papers/9f1c837b1b1df7adfcdd538c3698e332)
 
 StatsPAI is the **agent-native** Python package for causal inference and applied econometrics. One `import`, 390+ functions, covering the complete empirical research workflow — from classical econometrics to cutting-edge ML/AI causal methods to publication-ready tables in Word, Excel, and LaTeX.