Add CHANGELOG.md for v0.1.0 release

Document all features and changes in initial release including:
- Containerized Windmill workspace sync service
- Flask webhook server with API-based secret management
- GitHub workflow for automated Docker builds
- Comprehensive documentation and validation scripts

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project 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).
+
+## [Unreleased]
+
+## [0.1.0] - 2025-11-09
+
+### Added
+- Containerized service for syncing Windmill workspaces to Git repositories
+- Flask webhook server with `/sync` and `/health` endpoints
+- wmill CLI integration for pulling workspace content from Windmill
+- Automated Git commit and push functionality with PAT authentication
+- Docker container with Python 3.11, wmill CLI, Git, and Flask
+- Network isolation - service only accessible within Docker network (no host exposure)
+- Integration with existing Windmill docker-compose files
+- External volume support for persistent workspace data
+- Comprehensive documentation (README.md and CLAUDE.md)
+- MIT License
+- Docker build validation script (`scripts/validate_docker_build.sh`)
+- GitHub workflow for automated Docker image builds on version tags
+- GitHub Container Registry (GHCR) publishing support
+- Automated draft release creation for stable versions
+
+### Changed
+- Refactored security model: secrets now passed via JSON API payload instead of environment variables
+- Updated sync.py to accept configuration via function parameters rather than env vars
+- Enhanced server.py to parse and validate JSON payloads with required fields
+- Improved documentation to reflect API-based secret configuration model
+- Removed secret values from .env.example and docker-compose.yml
+
+### Security
+- Secrets (Windmill tokens, Git tokens) no longer stored in environment variables
+- All sensitive data managed by Windmill and passed per-request via JSON payload
+- Network-isolated design ensures service is only accessible within Docker network
+- PAT-based Git authentication using HTTPS (no SSH key management required)
+
+[Unreleased]: https://github.com/yourusername/windmill-git-sync/compare/v0.1.0...HEAD
+[0.1.0]: https://github.com/yourusername/windmill-git-sync/releases/tag/v0.1.0