Layout grid

Agents guess numbers. A guessed .padding(13) looks almost right and makes the whole app feel slightly off. This skill replaces guessing with one scale and three checks that fail when something is off.

The template has everything in place: Sources/<App>/UI/Grid.swift and tools/. For an app made before this skill, copy UI/Grid.swift, tools/grid-lint.sh, and tools/check-layout.swift from the new-mac-app skill's template, and the layout file line from its Snapshot.swift.