feat(changelog): add priority order for subcategory labels

When a PR has multiple content-type labels, use priority order:
1. breaking change (highest)
2. bugfix
3. feature
4. refactor (lowest)

Example: PR with 'bugfix' + 'feature' + 'refactor' → categorized as 'bugfix'

This prevents PRs from appearing in multiple subcategories and ensures
the most important change type is highlighted.

Author: MickLesk