Update agent list based on files

[jsburckhardt]

Describe the bug

I updated / did the "create new agent" finished it and saw it under .github/agents/newbestie.agent.md but I couldn't select it till I /exit and run copilot again.

Affected version

GitHub Copilot CLI 0.0.402.

Steps to reproduce the behavior

1. copilot
2. /agent -> "create new agent..." -> project
3. create with copilot || create manually
4. /agent

Expected behavior

I would expect to see the newly created agent in step 4

Additional context

No response