Custom agent selection reverts to Default agent mid-session

**Description**
When starting a session with a specific custom agent explicitly selected for the initial prompt, the agent selection reverts to the Default agent as the session progresses (during subsequent reasoning/turns).

**Steps to reproduce**
1. Start a new session/kickoff prompt with a specific custom agent selected.
2. Observe the initial prompt runs under the selected agent.
3. As the conversation continues and further thinking/turns happen, the session shows/reverts to the 'Default agent'.

**Expected behavior**
The explicitly selected custom agent should remain active for the duration of the session (or at least persist across turns) unless the user explicitly changes it.

**Actual behavior**
Agent selection reverts to Default agent after the initial prompt.

**Additional context**
Reported via Copilot CLI session. Unclear whether this is a UI display issue (mislabeling) or an actual functional reversion of agent config underneath.

首先，在使用所选自定义代理的 Copilot CLI 会话中重现该问题，然后在后续轮次中比较显示的代理与代理配置。跟踪会话和代理选择的入口点，以确定这种回退是功能性的还是仅仅是显示问题；当所选自定义代理在多个轮次中保持活动状态，并且该行为由回归测试覆盖时，即视为完成。

由索引模型根据 Issue 内容生成。