Skill validation requires 'license' in SKILL.md frontmatter (spec says optional)

[ezonno]

Summary

When using Copilot CLI skills, validation/loading appears to require a license: property in SKILL.md YAML frontmatter.

Per the Agent Skills spec, license is optional in the SKILL.md format:
https://agentskills.io/specification#skill-md-format

Expected behavior

A skill with valid frontmatter containing only required fields (e.g. name, description) should load/validate successfully without requiring license:.

Actual behavior

If license: is missing from SKILL.md frontmatter, the skill is rejected/flagged (or fails to load/validate).

Reproduction steps

1. Create a minimal skill directory.
2. Add SKILL.md with:

   ---
   name: example-skill
   description: "Example"
   ---

3. Attempt to add/load the skill via Copilot CLI.
4. Observe failure/warning about missing license.
5. Add a license: line (e.g. license: Apache-2.0 or license: Proprietary) and retry.

Environment

• Copilot CLI version: 0.0.374
• OS: Linux
• Date observed: 2026-01-06

Notes

The spec shows license: as an optional frontmatter field (and also allows descriptive values like license: Proprietary. LICENSE.txt has complete terms).