Split release prep from tag release

[ttak0422]

Summary

• add a separate release-prep.yml workflow that creates a release preparation PR from a manually supplied version
• simplify release.yml so tag pushes only create the GitHub Release and do not mutate the repository
• keep CHANGELOG.md updates on the normal PR review path instead of generating them after release

Release flow after this change

1. Run release-prep.yml with a version like 0.2.4
2. Review and merge the generated release prep PR
3. Tag the resulting merge commit as v0.2.4
4. Let release.yml create the GitHub Release from that tag

Why

The previous release workflow still treated changelog updates as a post-release concern. That avoided direct pushes to the default branch, but it kept the release pipeline split across tagging and a follow-up changelog PR.

This change makes main the source of truth for release metadata before tagging, which is more consistent with a PR-first repository policy.

Closes #19

Testing

• reviewed workflow YAML locally
• verified git-cliff --tag vX.Y.Z --output CHANGELOG.md behavior locally