Fix marketplace release sync push auth

[sf-jin-ku]

Summary

• keep SSH credentials available for the marketplace checkout so the workflow can push back to sendbird/codex-plugins
• add an optional workflow_dispatch release_tag input so an existing release can be re-synced manually
• make the workflow checkout the requested release tag and use that tag in the marketplace sync commit message

Testing

• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/release-sync.yml"); puts "ok"'
• git diff --check -- .github/workflows/release-sync.yml