docs: add coverage badge to README

Display test coverage percentage badge that links to
the detailed HTML coverage report on GitHub Pages.

Author: ryoppippi

README.md

@@ -2,6 +2,7 @@
 
 [![PyPI version](https://badge.fury.io/py/stackone-ai.svg)](https://badge.fury.io/py/stackone-ai)
 [![GitHub release (latest by date)](https://img.shields.io/github/v/release/StackOneHQ/stackone-ai-python)](https://github.com/StackOneHQ/stackone-ai-python/releases)
+[![Coverage](https://stackonehq.github.io/stackone-ai-python/badges.svg)](https://stackonehq.github.io/stackone-ai-python/html/)
 
 StackOne AI provides a unified interface for accessing various SaaS tools through AI-friendly APIs.