feat: changelog generation from Conventional Commits

[adamdaw]

Add automated changelog generation to the release workflow.

Context

The project enforces Conventional Commits via pre-commit hook, but does not generate a CHANGELOG.md from commit history. Tools like commitizen, auto-changelog, or git-cliff can parse Conventional Commits and produce a structured changelog.

Acceptance Criteria

• CHANGELOG.md generated on version tag push (release.yml)
• Groups by type (feat, fix, chore, docs)
• Includes commit hash and date
• Attached to GitHub Release alongside PDF/HTML/DOCX artifacts

Reference: docs/ENHANCEMENTS.md, conventionalcommits.org