feat: adopt frontend-design (Apache 2.0 fork) + progressive-disclosure extensions

Forked verbatim from anthropics/skills/skills/frontend-design (Apache 2.0).
LICENSE.txt preserved. Upstream SKILL.md prose (aesthetic guidance,
archetype list, anti-pattern callouts) kept intact.

Extensions added (clearly marked, load progressively — base SKILL.md
stays lean for simple cases):

SKILL.md:
  - Description extended with explicit negative triggers: narrow
    maintenance (single CSS bug, dependency upgrade, a11y-only retrofit),
    operational contexts where stakeholder has specified "minimal,
    functional, no creative direction," backend / API work, non-web UIs
    (mobile native, desktop, terminal), and refactoring without visible
    design component.
  - New "Workflow" section at the end of SKILL.md: four phases (intake,
    commitment, build, review) with pointers to reference files. Simple
    component tweaks skip the workflow; non-trivial redesigns walk it.
  - New "References" section: table mapping file → load-when condition.
  - Attribution footer marking upstream source + what's locally added.

references/workflow.md (~150 lines)
  Intake questions (purpose, audience, operational context, functional
  priority, technical constraints, brand references, success criteria).
  Commitment step (archetype pick, trade-offs, font pairing, palette,
  motion, layout as one-line decisions). Build reminders. Review
  pointer. Guidance on when to skip phases.

references/accessibility.md (~200 lines)
  WCAG AA contrast thresholds + practical check guidance. Keyboard
  navigation + focus management. Semantic HTML + ARIA rules. Reduced-
  motion CSS snippet. Smoke checklist. Operational-context note for
  defense / ISR work.

references/responsive.md (~160 lines)
  Mobile-first vs desktop-first decision. Named breakpoints (Tailwind-
  style) vs magic pixels. Container queries. Aesthetic translation
  table — how each archetype handles small-screen scaling. Responsive
  typography with clamp(). Operational-dashboard note: desktop-primary
  is a legitimate product decision.

references/design-review.md (~170 lines)
  Archetype check (does the build read as what was committed to?).
  Anti-pattern grep for fonts, palette, layout, motion, backgrounds,
  components. Code-quality-match check (ornate design + lazy code =
  failure). Performance sanity. Convergence check (if last 3 builds
  all used the same archetype, break the pattern). The one-sentence
  test for memorability.

references/rationale-template.md (~160 lines)
  Template for design-rationale.md alongside the build. Nine sections
  (purpose, archetype, locked decisions, deliberately absent,
  accessibility, responsive, implementation, open questions,
  references). Filled example using a DeepSat SOCOM demo landing page
  to show density and specificity.

Structure matches Anthropic's own pdf / docx / webapp-testing pattern
(SKILL.md entry + references/ for progressive disclosure). Makefile
SKILLS extended; make install symlinks globally.

Adoption caveat resolved: name kept as `frontend-design` (not renamed
to ui-design) — "frontend" signals scope (web code, not mobile /
desktop / terminal UIs), upstream parity preserved for attribution.

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