In the chat prompt, you referred to @workspace, which is called a chat participant. Participants are domain experts that can perform tasks or answer questions in a specific domain. Specifically, @workspace knows about VS Code workspaces and your codebase.
GitHub Copilot
Copilot in the CLI
GitHub Copilot Chat
@workspace
をchatに書くことで、ワークスペース全体のコンテキストを参照できる。