gh-120: Add JOSS paper to the repository

[Copilot]

🚀 Pull Request Checklist

✅ Summary

Adds a JOSS draft PDF build workflow and empty paper template, mirroring the setup in cloelib.

🔄 Changes

• .github/workflows/joss.yml — workflow using openjournals/openjournals-draft-action@v1.0, triggered on changes to paper/
• paper/paper.md — empty JOSS frontmatter skeleton
• paper/paper.bib — empty bibliography file

🛠 How to Test

Push or open a PR with changes under paper/ — the workflow will produce a draft PDF artifact.

📝 Documentation

• This PR does not require documentation changes

🏗 Related Issues

Closes #120

📸 Screenshots (if applicable)

N/A

📌 Additional Notes

Paper template is intentionally empty per task requirements. Content to be added in a follow-up.

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✅ PR Checklist for Developers

• I have titled this PR before merging as "gh-#:", where "#" represents the task it closes
• I have run locally pre-commit using pre-commit run --all-files
• I have tested my changes locally
• No new warnings or errors introduced
• I have updated documentation (if applicable)
• My changes do not introduce breaking changes
• I have added tests (if applicable)
• I have consistently updated the GitHub information for the project, including milestones, task types, and other relevant details.

✅ PR Checklist for Reviewers

• The next PR targets the correct branch
• CI tests have run and passed for the latest commit on the source branch
• Check that the code can still be installed if new packages are imported
• If necessary, the notebooks in the playground will be updated in a corresponding follow-up PR
• Coverage percentage is retained or increased
• Quality of new/changed code is acceptable
• Quality of new/changed unit tests is acceptable
• No data files have been included in the commits
• Implementation follows the agreed task description point by point
• Check that there are no No newline at the end of file warnings
• Check that any added folder/file has been added to the README.md file
• Check that the documentation has been updated accordantly
• Check that the corresponding branch has been deleted after merging. If not, delete it