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/h5-letter-hero.md

@@ -0,0 +1,11 @@
+### H5 · Letter Hero
+First-person opening — "Dear reader,". No buttons in fold. Reads as personal correspondence.
+*Use when:* the founder's voice is the brand.
+*Don't confuse with:* H1 Marquee (which is impersonal declaration).
+
+```html
+<section class="hero-letter">
+  <p class="salutation"><em>Dear reader,</em></p>
+  <p class="lede">…</p>
+</section>
+```