Builds the Claude Code skill that teaches an agent to write LineaUI markup.

Everything comes from the compiled components (__components__/0 and their docs), so the skill describes exactly the installed version: SKILL.md is a short index with the rules that hold for every component, and references/<component>.md holds the attrs, slots and examples of one component, read only when that component is used.

mix linea_ui.install writes it when it sets the app up and mix linea_ui.gen.skill refreshes it after an upgrade.