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/n3-side-rail.md

@@ -0,0 +1,14 @@
+### N3 · Side-rail
+A thin vertical strip on the left edge — wordmark rotated, plus 2–3 dot-indicators for sections. Editorial / portfolio energy.
+*Use when:* the page is long and section-numbered.
+*Don't confuse with:* N1 Top wordmark (which is horizontal).
+
+```html
+<nav class="nav-rail">
+  <p class="wordmark vertical">Studio</p>
+  <ul class="dots"><li></li><li></li><li></li></ul>
+</nav>
+```
+```css
+.nav-rail { position: fixed; left: 0; top: 0; bottom: 0; width: 3rem; padding: var(--space-md); writing-mode: vertical-rl; }
+```