docs: document change file comment format

[janechu]

Pull Request

📖 Description

Adds guidance to the shipping skill for Beachball change-file comment formatting.

The new guidance explains that change-file comments are release-note text rather than commit messages, so they should be concise sentence-style comments instead of conventional-commit-prefixed strings.

👩‍💻 Reviewer Notes

This is documentation-only guidance for AI agents and contributors using the FAST shipping skill.

📑 Test Plan

• git diff --check -- .github/skills/shipping/SKILL.md

✅ Checklist

General

• I have included a change request file using $ npm run change
• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.