Allow "compatibility" property in frontmatter for consistency with Agent Skills Specification

The quick_validate.py script enumerates a set of five allowed_properties for the YAML frontmatter: https://github.com/openai/skills/blob/2c9ebb321958f64aa9414b46e4240901e2b9e1a9/skills/.system/skill-creator/scripts/quick_validate.py#L40

The Agent Skills Specification lists six properties for the frontmatter (https://agentskills.io/specification#frontmatter-required).

The compatibility field is optional per the specification (https://agentskills.io/specification#compatibility-field) but disallowed by the quick_validate.py script.

The set of allowed_properties should align with the specification.