Add changelog CI job with dedicated commit enforcement

[dannywillems]

Problem

No CI enforcement that CHANGELOG.md changes are in dedicated commits. Mixed commits (code + changelog) make git history harder to follow and revert.

Solution

• Add `.github/workflows/changelog.yaml` with:
  • `tarides/changelog-check-action@v3` for PR changelog presence
  • Custom script `.github/scripts/check-changelog-commit.sh` that verifies:
    1. CHANGELOG.md changes are in their own commit
    2. Referenced commit hashes exist
    3. Commit link URLs match their reference keys
• Add shellcheck step for the script