Drop babel and gulp

babelとgulpをやめる

- babel: ネイティブでESMなどを扱えるため変換は不要なはず
- gulp: babelのために使ってるので不要

## TODO

- babelとgulpの変換フローを調査
- 削除
- cleanup

developmentとビルドでそれぞれ使い方が異なるので、それぞれ対応数ｒ

## TEST

- ビルドが通る
- テストが通る
- npm startでちゃんと起動する

Start by tracing the Babel and Gulp conversion flow, checking how development and build usage differ. Remove both tools and clean up their flow without changing the intended behavior. Done means the build passes, tests pass, and npm start launches successfully.

Written by the indexing model from the issue text.