ci: add release promotion automation

[MioYuuIH]

Summary

• add a manual workflow to open or reuse a develop -> main promotion PR
• add release-please on main as the release conductor for release PRs, tags, and GitHub Releases
• update the offline bundle workflow comment to expect release automation instead of manual release creation

Validation

• yamllint .github/workflows/promote-develop-to-main.yml .github/workflows/release-please.yml .github/workflows/pack-bundle.yml
• git diff --staged --check before commit

Notes

• Configure RELEASE_AUTOMATION_TOKEN with a GitHub App token or fine-grained PAT if release-please-created tags/releases must trigger downstream workflows.
• actionlint is not installed in the local environment, so GitHub Actions expression validation will run in CI.