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

[jiaczh]

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