[Bug] Sub-agent model change requires application restart to take effect

提交前确认 · Pre-submission checklist

• 我已搜索过现有 issue，确认这不是重复 / I searched existing issues and confirmed this isn't a duplicate.
• 我已阅读 CONTRIBUTING.md / I've read CONTRIBUTING.md.

问题类别 · Category

对话 / Agent 交互 · Agent chat

涉及的 Agent 框架 · Agent framework

ZCode Agent（自研）

严重程度 · Severity

影响体验 · Major (功能可用但体验受损 / works but degraded)

复现频率 · Reproducibility

必现 · Always

问题描述 · Description

When changing the model of a sub-agent, the UI updates to reflect the new choice, but the system context is not re-initialized. It permanently locks onto the first loaded model and continues to use it until ZCode is completely restarted.

复现步骤 · Steps to reproduce

1. Set the main agent to Model X.
2. Set a sub-agent to Model Y.
3. Trigger any action to initialize and use the sub-agent with Model Y.
4. Go to settings and change the sub-agent's model to Model Z.
5. Observe that the UI correctly displays Model Z as selected.
6. Trigger the sub-agent action again.

期望表现 · Expected behavior

The system should dynamically re-initialize the sub-agent and execute the request using Model Z.

实际表现 · Actual behavior

The system continues to execute the request using Model Y (the change is purely visual in the UI).
Note: This was diagnosed because Model Y had an exhausted quota, causing subsequent requests to fail even after switching to Model Z. If the old model has a valid quota, it simply keeps running it silently.

ZCode 版本 · ZCode version

lastest + olders

设备 / 系统 / 浏览器 · Device / OS / Browser

Linux Mint

截图 / 录屏 / 日志 · Screenshots / Recordings / Logs

No response

Reproduce the issue by initializing a sub-agent with Model Y, changing it to Model Z in settings, and triggering the action again. Trace the sub-agent model selection and initialization path; it is done when subsequent requests use Model Z without restarting ZCode.