`skill-creator` skill incorrectly prohibits optional frontmatter fields allowed by Agent Skills spec

The `skill-creator` skill's SKILL.md contains guidance that contradicts the official [Agent Skills specification](https://agentskills.io/specification#frontmatter-required).

## Current behavior

Line 313 of the skill-creator SKILL.md states:
> "Do not include any other fields in YAML frontmatter."

https://github.com/anthropics/skills/blob/main/skills/skill-creator/SKILL.md#frontmatter

This implies only `name` and `description` are allowed.

## Expected behavior

Per the official Agent Skills specification, the following fields are **optional** and should be permitted:

- `license` - License name or reference to a bundled license file
- `compatibility` - Environment requirements (max 500 characters)
- `metadata` - Arbitrary key-value mapping for additional metadata
- `allowed-tools` - Space-delimited list of pre-approved tools (experimental)

## Suggested fix

Update the skill-creator SKILL.md to align with the official specification by:
1. Documenting the optional fields (`license`, `compatibility`, `metadata`, `allowed-tools`)
2. Changing the guidance from "Do not include any other fields" to something like "Only use fields defined in the Agent Skills specification"

## Reference
- Official spec: https://agentskills.io/specification#frontmatter-required