--agent <name>  doesn't recognize custom agents loaded via  --add-dir

Describe the bug

Custom agents loaded from a directory passed via  --add-dir  are correctly discovered for task-tool delegation (they appear in the model's list of available sub-agents, and can be invoked by mentioning them by name in a prompt), but the top-level  --agent   flag does not recognize them. Passing  --agent   for an agent that only exists under an  --add-dir -added directory's  .github/agents/  fails with "No such agent", even when that agent's frontmatter sets  user-invocable: true .

Affected version

GitHub Copilot CLI 1.0.83

Steps to reproduce the behavior

1. Create a custom agent at  /.github/agents/my-agent.agent.md  with  user-invocable: true  in its frontmatter.
2. Run:
   copilot --add-dir <some-dir> -p "List every custom agent name available to you via the task tool." --allow-all-tools
   → my-agent appears in the list, confirming --add-dir loaded it correctly.
3. Run:
   copilot --add-dir <some-dir> --agent my-agent -p "hello" --allow-all-tools
   → Fails with:
   No such agent: my-agent, available: <only agents registered through other mechanisms, e.g. plugins>

Expected behavior

 --agent   should be able to select any agent discovered via  --add-dir 's  .github/agents/  directory — the same set of agents already visible to/usable by the task tool in that same session — especially when the agent explicitly sets  user-invocable: true .

Additional context

• Workaround: instead of  --agent  , invoke the agent by mentioning its name directly in the prompt, e.g.  -p "use the my-agent agent to ..." . This reliably delegates to the correct agent via the task tool.
• OS: macOS
• Shell: bash

首先，使用 frontmatter 设置为 user-invocable: true 的 .github/agents/my-agent.agent.md 文件，通过 --add-dir 和 --agent 重现该行为。比较 task tool 发现该 agent 的方式与顶层 --agent 选项验证名称的方式。完成标准是：--agent my-agent 能够选择同一个自定义 agent，且不会出现“No such agent”错误。