fix(INFRA-3081): implement duplicate detection for all commit types

- Add duplicate detection for both PR-based and direct commits
- For PR commits: check if PR number already exists in changelog
- For direct commits: check if description already exists in changelog
- This fixes idempotency issues where direct commits like 'Update Attributions'
  were being re-added on every workflow run

Fixes INFRA-3081

BREAKING CHANGE: None - this is backward compatible
The behavior change is that direct commits (without PR numbers) are now
included in changelogs by default, but duplicates are properly detected
and excluded on subsequent runs.

Future PR will add optional --require-pr-numbers flag for teams that want
to filter out direct commits during generation.

Author: Qbandev