Custom agents intermittently unavailable in new sessions

从 /chat 的会话启动流程和从 .github/agents 发现自定义代理开始，重现新会话中间歇性出现的 Unknown agent_type 结果。比较成功和失败的会话，然后验证发现过程是否能在接受工作之前可靠完成，或者是否会显示明确的失败信息和恢复操作。

Title: Custom agents intermittently unavailable in new sessions — suspected startup race

Description:

Custom agents defined in my repository’s .github/agents/ directory intermittently fail to register when starting a session in the GitHub Copilot desktop app.

In affected sessions, dispatching a custom agent returns Unknown agent_type, even though its definition exists in the repository. Other sessions correctly expose all 28 custom agents. This appears to be a session initialization or agent-discovery problem; a startup race is suspected but not confirmed.

Steps to reproduce:

1. Open a repository containing custom agents under .github/agents/.

2. Start a new session in the Copilot app.

3. Ask the agent to invoke one of those custom agents.

4. Intermittently, dispatch rejects the custom agent with Unknown agent_type. Other sessions load the agents successfully.

Expected behavior:

Custom-agent discovery should complete before the session accepts work requiring those agents. If discovery fails, the app should show an explicit error and recovery action instead of allowing execution to continue with missing capabilities.

Actual behavior and impact:

Sessions can start without the repository’s custom agents being available. Automated work then fails or continues without the intended specialist agents. This makes workflows unreliable and forces me to check agent availability and restart sessions manually.

Environment:

• GitHub Copilot desktop app on macOS

• App version: 1.1.21

• Repository opened in a local worktree

• Intermittent; exact trigger and root cause unknown

Please investigate session-start ordering and custom-agent registration, and provide a clear indication when discovery has failed or is incomplete.