docs: add initial CHANGELOG.md file

Document all notable changes and features for Unity Code MCP Server in a structured format. This helps users track updates and improvements to the project.

Author: hackerzhuli

CHANGELOG.md

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+# Changelog
+
+All notable changes to Unity Code MCP Server will be documented in this file.
+
+## [1.0.0] - 2025-7-23
+
+### Added
+- Initial release of Unity Code MCP Server
+- Asset database refresh tool with compilation error reporting
+- Test execution tool with comprehensive test results and logs
+- Compilation-resilient architecture that survives Unity domain reloads
+- Self-contained Rust binary with no runtime dependencies
+- Support for Unity 6.0 and higher
+- Cross-platform compatibility (Windows, macOS, Linux)
+
+### Features
+- **Coding-focused design**: Only essential tools for AI-driven code development
+- **High performance**: Built in Rust for speed and minimal resource usage
+- **Robust error handling**: Graceful handling of Unity compilation cycles
+- **Comprehensive test support**: Both EditMode and PlayMode test execution