Organization-level custom agents not visible in Copilot CLI when no GitHub-hosted repository is in the working directory

### Describe the bug

Organization-level custom agents defined in the agents/ directory of the org's .github-private repository do not appear in gh copilot CLI (via /agent) unless the CLI is launched from a directory that contains a git repository with a GitHub remote belonging to that organization. This works perfectly fine in VS Code.

### Affected version

_No response_

### Steps to reproduce the behavior

1. Define org-level custom agents in /.github-private/agents/.
2. On a machine with no local clones of any GitHub repository in that org, run copilot from any directory.
3. Run /agent — org-level agents are not listed.

### Expected behavior

Org-level agents should be resolved from the authenticated user's organization membership, not from the git remote of the current working directory.

### Additional context

Our organization uses Azure DevOps for source control and only has the .github-private repository on GitHub. As a result, org-level agents are inaccessible in the CLI without maintaining an otherwise unused GitHub repo clone on every developer machine.

まず、GitHub リポジトリも remote もないディレクトリで問題を再現し、次に gh copilot CLI で /agent コマンドを実行して、GitHub がホストするリポジトリのディレクトリと比較します。認証済みユーザーの組織メンバーシップから CLI が組織レベルのエージェントを解決する方法を追跡し、組織の .github-private/agents/ ディレクトリにあるエージェントが、現在の git remote に依存せずに表示されることを検証します。

索引モデルが issue の本文から書いたものです。