--agent <agent name> does not work with --plugin-dir <dir> -p <prompt>

### Describe the bug

When I use --agent with --plugin-dir -p , it starts scanning the .copilot or .github directory for .
Though this agent is present in the plugin-dir, copilot is not able to find it.

**If I don't pass -p , the copilot TUI opens and I can see the agent selected.**

I am using the version 1.0.31 (latest)

### Affected version

_No response_

### Steps to reproduce the behavior

1. Download a plugin to a directory, with a "agent/.agent.md"
2. pass the same agent name as "copilot --agent --plugin-dir " -> You will see the TUI showing "agent name" selected
3. Now pass with -p switch "copilot --agent --plugin-dir -p 'hi' " -> This will error out saying the agent is not available

### Expected behavior

(With and without -p) and with --plugin-dir and with --agent, copilot should be able to find the right agent

### Additional context

_No response_

Reproduce the issue with a plugin containing agent/.agent.md, first without -p and then with `copilot --agent --plugin-dir -p 'hi'`. Trace how agent discovery differs between the TUI and prompt modes; done means the plugin agent is found and selected in both invocations.

Rédigé par le modèle d'indexation à partir du texte de l'issue.