Review request: guidance for existing design systems in frontend-design skill

Opening this to flag #1561 for maintainer review.

The PR adds a "Working inside an established design system" section to the `frontend-design` skill. Context: the skill currently assumes a blank-canvas brief where inventing a bespoke palette/typeface is the goal, but that doesn't hold when the brief names, or the codebase already uses, an established design system (e.g. Adobe Spectrum, Material, Fluent). The addition covers when to switch modes and uses Adobe Spectrum as a concrete, verified reference case (its stated principles, and its canonical token/component sources).

This is my first contribution to this repo — happy to make any changes the maintainers want.

PR: #1561