Add a skill for rushstack

[L-Qun]

Previously, we wanted AI Agents to better understand RushStack knowledge and best practices, so we open-sourced Rush rules: https://github.com/microsoft/rushstack/blob/main/.cursor/rules/rush.mdc.

This helps Agents better understand RushStack related best practices. Generally, this would be added to the system prompt as a one-time inclusion, which means wasting a lot of context. And the best answer to this problem has emerged — Skills.

Skills were recently proposed by Claude and have become very popular. So I decided to open-source the RushStack skill so that it can better help AI agents work within a Rush monorepo

[octogonz]

Let's add a CHANGELOG.md in this folder. It's apparently not part of the "skills" standard, but it seems like a major oversight, which we can expect them to correct eventually. 😊

[octogonz]

(Also, we should find a way to publish skills to a package system like NPM. Otherwise, how do we designate which Git commit is "published" versus "not ready for usage yet"? How do you efficiently fetch skills without hammering github.com? How does an enterprise review them? There's lots of important reasons why nobody installs TypeScript libraries directly from github.com.)