Create or update the changelog for this repository.

Your task:
1. If no CHANGELOG.md exists, create one
2. If it exists, add a new entry for recent changes
3. Use standard changelog formatting

Format:
```markdown
# 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]

### Added
- New features added

### Changed
- Changes in existing functionality

### Deprecated
- Soon-to-be removed features

### Removed
- Removed features

### Fixed
- Bug fixes

### Security
- Security fixes

## [1.0.0] - YYYY-MM-DD

### Added
- Initial release
```

Document the tasks and changes made since the last session. Use semantic versioning and standard categories.