docs: add initial CHANGELOG.md file for Unity Code extension

This file documents all notable changes following the Keep a Changelog format and Semantic Versioning. It includes the initial release features and technical requirements.

Author: hackerzhuli

CHANGELOG.md

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+# Changelog
+
+All notable changes to the Unity Code extension will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [1.0.0] - 2025-07-04
+
+### 🎉 Initial Release
+
+First stable release of Unity Code - bringing Unity IDE capabilities to VS Code-based code editors.
+
+### Added
+- 🧪 **Unity Test Explorer** - Run Unity tests directly in editor with inline results
+- 📊 **Unity Console Integration** - Real-time logs with clickable stack traces
+- 🐛 **Integrated Debugger** - Unity debugging with MonoDebugger
+- 📚 **Intelligent Documentation** - Hover docs with Unity API and .NET links
+- 🔍 **Advanced Code Analysis** - Roslyn-powered Unity-specific static analysis
+- 🔄 **Smart Asset Management** - Automatic meta file handling and asset refresh
+- 🤖 **AI Editor Support** - Full compatibility with Cursor, Windsurf, and Trae
+
+### Technical
+- Requires companion Unity package: [Visual Studio Code Editor](https://github.com/hackerzhuli/com.hackerzhuli.code)
+- Free and open source with no licensing restrictions
+
+---
+
+**Unity Code 1.0.0** - Where Unity development meets modern AI-powered editors! 🎮✨