[release tool]: merge staging pull requests through the tool itself

Currently it is unclear what should be used to merge staging pull requests. We should just let the release tool merge the PR as it knows exactly where the PR should go into. The merge tool would rely on target labels which might not work temporarily e.g. when branching-off a feature freeze.

We should either merge automatically once the PR is approved + checks are passing, or we just add a double-prompt that can be submitted to manually start the merging (useful in case the PR checks are not significant as in COMP and FW - though worth noting that this could be done through a configuration as well in `.ng-dev`.)

The cherry-pick PRs should be merged automatically based on our discussion

まず、リリースツールの staging pull request のマージフローを特定し、issue で言及されている `.ng-dev` 設定を確認します。自動マージを、提案されている二重プロンプトおよび設定オプションと比較し、記載されている cherry-pick の動作も含めます。説明されている staging と cherry-pick のケースについてマージポリシーが決定され、実装されれば完了です。

索引モデルが issue の本文から書いたものです。