Clarify license inconsistency between CONTRIBUTING and license notice

## Title

Clarify license inconsistency between CONTRIBUTING and license notice

## Description

Hi GitHub Copilot App maintainers,

I noticed a potential inconsistency between the contributing guidelines and the license information in this repository.

In the **Contributing** section, it says:

> Contributions to this project are released to the public under the project's open source license.

However, the license information appears to state only:

> © GitHub, Inc. All rights reserved.

This is confusing because **“All rights reserved”** does not appear to indicate an open source license, while the contributing guidelines refer to the project’s open source license.

I am particularly interested in this clarification because I work for a large enterprise and need to understand the app’s licensing terms clearly before evaluating its use or making any contribution.

Could you please clarify:

- Which license applies to this project?
- Under which license are contributions accepted?
- Is this repository intended to be open source?
- If an open source license applies, could a corresponding `LICENSE` file or license notice be added or updated?

This clarification would help contributors and enterprise users understand the legal terms before submitting issues, pull requests, code contributions, or evaluating the app for use.

Thank you!

まず CONTRIBUTING セクションとリポジトリのライセンス通知を読み、その後、LICENSE ファイルまたはその他のライセンスメタデータが存在するか確認します。maintainer とプロジェクトおよびコントリビューションに適用されるライセンスを確認します。完了とは、ドキュメントにその条件が一貫して記載され、必要な LICENSE ファイルまたは通知が更新されていることです。

索引モデルが issue の本文から書いたものです。