feat(skills): hand-crafted design.html for each template

Each template gets a design.html that renders the design system: cover,
manifesto, palette, typography, surface, motion, background shader, do/don't
guidelines, and a templates index. Used by the design picker fine-tune phase
to preview the chosen template's design system at full fidelity.

design.html shares the --primary / --secondary / --tertiary / --accent token
contract with the picker — the picker re-themes each design.html iframe at
render time by overriding those tokens.

Author: vanceingalls