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.

Start by reproducing the issue from a directory without a GitHub repository or remote, then run the /agent command in gh copilot CLI and compare it with a GitHub-hosted repository directory. Trace how the CLI resolves organization-level agents from the authenticated user's organization membership and verify that agents in the organization's .github-private/agents/ directory appear without relying on the current git remote.

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