Agent Skills: using-xtool

This skill should be used when building iOS apps with xtool (Xcode-free iOS development), creating xtool projects, adding app extensions, or configuring xtool.yml. Triggers on "xtool", "SwiftPM iOS", "iOS on Linux", "iOS on Windows", "Xcode-free", "app extension", "widget extension", "share extension". Covers project setup, app extensions, and deployment.