ci: use reusable calens workflow from reusable-workflows

[DeepDiver1975]

Summary

• Replaces the inline changelog job with a call to owncloud/reusable-workflows/.github/workflows/calens.yml@main
• Trigger (pull_request, push to master/tags) is unchanged
• On pull requests and tags the reusable workflow runs calens and shows the diff but skips PR creation (guarded by github.ref == refs/heads/master)

Test plan

• Merge ci: add reusable calens changelog workflow reusable-workflows#39 first
• Open a PR in this repo — confirm the changelog job runs but does not create a changelog PR
• Push to master — confirm a chore/changelog-update PR is created/updated

🤖 Generated with Claude Code

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.

[DeepDiver1975]

owncloud/reusable-workflows#39 merged