general-purpose subagent uses gpt-5.4-mini even when configured to inherit model

### Describe the bug

general-purpose subagent uses gpt-5.4-mini even when configured to inherit model from session.

### Affected version

1.0.75

### Steps to reproduce the behavior

1. Configure GPT-5.6 Sol as the session model.
2. Configure general-purpose subagent to inherit model.
3. Trigger a general-purpose subagent.
4. Subagent uses gpt-5.4-mini.

### Expected behavior

Subagent follows configuration, inheriting session model.

### Additional context

Operating system: Linux
CPU architecture: x86_64
Terminal emulator: Gnome Terminal
Shell: zsh

首先，使用列出的 session 和通用 subagent 配置复现问题，然后跟踪 CLI 中通用 subagent 的模型选择和继承处理。完成的标准是：当 subagent 的配置设置为 inherit 时，它会使用已配置的 session 模型，并且有一个覆盖所报告行为的回归测试。

由索引模型根据 Issue 内容生成。