dx: Add automated CHANGELOG.md generation from conventional commits

[nanaf6203-bit]

Description

The project uses conventional commits but has no automated changelog. Set up changelog generation from commit history.

Acceptance Criteria

• Add git-cliff or similar tool configuration
• Create cliff.toml with conventional commit mapping
• Group changes: Features, Bug Fixes, Performance, Documentation, Security, Testing
• Add npm script or make task: generate-changelog
• Add GitHub Actions workflow to auto-update CHANGELOG on release
• Document changelog generation process
• Generate initial CHANGELOG.md from existing commit history