Support agent discovery from current working directory (nested .github/agents/)

### Problem

When running `copilot` from a subdirectory that has its own `.github/agents/` folder (e.g., a monorepo with team-specific agent configs), only agents from the **git repo root's** `.github/agents/` are discovered. Agents defined in the cwd's `.github/agents/` are invisible to `/agent`.

### Use Case

In large monorepos, teams maintain specialized agents in subdirectories:

```
repo-root/
├── .github/agents/ ← these show up in /agent
│ ├── Bvt.agent.md
│ └── FleetUpgrade.agent.md
└── src/team-subfolder/
└── .github/agents/ ← these do NOT show up
└── my-custom-agent.agent.md
```

The CLI already respects cwd for `copilot-instructions.md` and instruction files. Agent files should follow the same pattern.

### Expected Behavior

Running `/agent` while cwd is `src/team-subfolder/` should show agents from **both**:
1. The git repo root `.github/agents/`
2. The cwd's `.github/agents/` (if different)

### Proposed Solution

Discover `.agent.md` files from both the git root and the current working directory's `.github/agents/`, similar to how custom instructions are resolved. If there are name conflicts, cwd agents could take precedence (or be prefixed with a path indicator).

### Environment

- Copilot CLI version: 0.0.421
- OS: Windows 11
- Shell: PowerShell 7