YAML Formatting VSCode Settings

Different people may have different default YAML formatters configured in their user settings. These may even fight each other in multi-author PRs and similar. We should decide on one and put it in the repo settings.

## Acceptance Critera
- `.vscode/settings.json` includes a default YAML formatter
- Recommended extension settings includes the extension used if applicable

Start by inspecting .vscode/settings.json and the repository's recommended extension settings. Decide on a single YAML formatter and record its default setting and extension recommendation there. Done means the acceptance criteria are satisfied and the repository consistently identifies one formatter.

索引モデルが issue の本文から書いたものです。