Skill `model` frontmatter field does not trigger automatic model switching

## Summary

When a skill's SKILL.md has a model field in its frontmatter (e.g. model: opus), the Claude Code harness does not switch to the specified model when the
skill is invoked. The model remains unchanged throughout the skill execution.

## Steps to Reproduce

1. Create a skill with model: opus in its frontmatter:

```yaml
---
name: my_discuss
model: opus
description: ...
---

1. Start a Claude Code session on a non-Opus model (e.g. Sonnet 4.6)
2. Invoke the skill via /my_discuss
3. Check the active model — it remains Sonnet 4.6; no switch to Opus occurs

Expected Behavior

The harness reads the model field and switches to the specified model before executing the skill.

Actual Behavior

The model is never switched. The skill runs on whatever model was active in the session before invocation.

Environment

• Claude Code version: 2.1.195
• OS: macOS (Darwin 25.5.0)
• Session model at time of invocation: Sonnet 4.6
• Skill frontmatter: model: opus

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