feat: add hallmark design skill

Anti-AI-slop design skill (greenfield pages, audits, redesigns, design
extraction from URLs/screenshots) -- installed and validated in an
earlier session, was sitting untracked. Full reference library:
components, macrostructures, themes, genres, verbs (audit/redesign).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

skills/hallmark/references/components/h2-split-diptych.md

@@ -0,0 +1,15 @@
+### H2 · Split Diptych
+Headline + lede on one side, image or product capture on the other. 6/6 or 7/5 columns.
+*Use when:* you can pair every claim with a visual proof.
+*Don't confuse with:* H6 Photographic (which puts the image full-bleed, not paired).
+
+```html
+<section class="hero-split">
+  <div><h1>…</h1><p>…</p><a class="cta-outline">…</a></div>
+  <figure><img src="" /></figure>
+</section>
+```
+```css
+.hero-split { display: grid; grid-template-columns: 7fr 5fr; gap: var(--space-2xl); align-items: center; }
+@media (max-width: 56rem) { .hero-split { grid-template-columns: 1fr; } }
+```