Support discovering custom agents from subdirectories in monorepos

### Describe the feature or problem you'd like to solve

Copilot CLI does not discover custom agents from a subdirectory’s .github/agents folder in monorepos, even when CLI is started from that subdirectory. The same agent works in VS Code and other CLI customizations (prompts, instructions, skills) in that subdirectory can still be recognized.

### Proposed solution

Allow Copilot CLI to discover custom agents from the current working directory’s .github/agents folder, not only from the Git repository root. This would make custom agent behavior more consistent with prompts, instructions, and skills, improve monorepo support, and reduce confusion when VS Code and Copilot CLI behave differently.

### Example prompts or workflows

root - .git
- app1
- .github
- agents
- app2
- .github
- agents

### Additional context

_No response_

Start by tracing how Copilot CLI discovers custom agents from the Git repository root, then compare that behavior with the existing discovery of prompts, instructions, and skills in a subdirectory. Verify the behavior from a monorepo subdirectory containing .github/agents, and consider the work complete when agents there are discovered consistently with the other customizations.

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