Skills with `disable-model-invocation: true` cannot be invoked in Desktop

### Short summary

Custom skills containing `disable-model-invocation: true` appear in Copilot Desktop Settings but are excluded from the runtime skill catalog and cannot be invoked. Removing the property immediately restores normal behavior.

### Affected version or release

v1.1.8

### Installation context

Custom user skill installed locally via the GitHub Copilot Desktop Skills feature. Skill location: C:\Users\WaldrBR\.agents\skills\thermo-nuclear-code-quality-review\SKILL.md The skill appears in Copilot Desktop Settings and is enabled. It also works correctly when invoked from GitHub Copilot CLI. The issue only occurs in Copilot Desktop, where the skill is excluded from the runtime skill catalog when `disable-model-invocation: true` is present in the skill frontmatter.

### What happened?

A custom skill was installed and enabled in GitHub Copilot Desktop but could not be invoked. Copilot Desktop reported that the skill was not installed, even though it appeared in Settings and worked correctly in GitHub Copilot CLI. Investigation determined that the issue was caused by the `disable-model-invocation: true` property in the skill frontmatter. Removing the property immediately restored normal behavior.

This started yesterday when I installed an update to github copilot cli (not sure what version). I was requested to update the app again today (was hoping it would fix the issue) & it didn't. The version listed above was installed today. Skill works with github copilot cli. I have had the skill installed for a long time & using it. The skill is installed from `https://www.skills.sh/cursor/plugins/thermo-nuclear-code-quality-review`

### Steps to reproduce

1. Create or modify a custom skill with the following frontmatter:

---
name: thermo-nuclear-code-quality-review
description: Test skill
disable-model-invocation: true
---
2. I installed with `npx skills add https://github.com/cursor/plugins --skill thermo-nuclear-code-quality-review1. I can provide the SKILLS.md file if needed because you don't want to run that command.

3. Ensure its enabled the skill in GitHub Copilot Desktop.

4. Verify the skill appears in the Skills Settings page.

5. Start a new chat and attempt to invoke the skill.

6. Observe that Copilot Desktop reports the skill is not installed or cannot be found.

7. Remove the `disable-model-invocation: true` property.

8. Restart GitHub Copilot Desktop and invoke the skill again.

9. Observe that the skill is now available and functions normally.

### Expected behavior

The skill should be available for invocation in GitHub Copilot Desktop when it is installed, enabled, and visible in the Skills Settings page. If `disable-model-invocation: true` is unsupported or invalid, Copilot Desktop should display a clear validation error rather than silently excluding the skill and reporting it as not installed.

### Additional context