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/n4-hidden-behind-k.md

@@ -0,0 +1,9 @@
+### N4 · Hidden behind ⌘K
+No visible nav. The user opens a command palette via `⌘K` to get anywhere. Designed for keyboard-first audiences.
+*Use when:* the page is for technical users who expect this affordance.
+*Don't confuse with:* N2 Floating chip (which is visible always).
+
+```html
+<button class="kbd-hint">⌘ K</button>
+<dialog class="palette">…</dialog>
+```