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

Start by reproducing the issue with the listed session and general-purpose subagent configurations, then trace the general-purpose subagent's model-selection and inheritance handling in the CLI. Done means the subagent uses the configured session model when its configuration is set to inherit, with a regression test covering the reported behavior.

Written by the indexing model from the issue text.