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/macrostructures/18-portfolio-grid.md

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+## 18 · Portfolio Grid
+
+Filterable cards of projects. Studio or designer homepages where the work is the product.
+
+- **Heading:** short tagline above the grid; no display.
+- **Body:** responsive grid of project cards, all same size or with subtle size variation.
+- **Divider:** filter bar above the grid; no internal rules.
+- **Button:** card-internal "View case study"; no global CTA.
+- **Image:** thumbnail per card.
+- **Reveal:** card-fade on filter change.
+
+Reach for it for design studios, agencies, photographer portfolios, any creative business where work is the pitch.
+
+Avoid for products. Portfolio Grid is service-business shape.
+
+Reference: Pentagram, 14islands, Locomotive, Bureau Borsche.
+
+**Sample opening lines** (imitate the *specificity* — Portfolio Grid openings name the volume and the era):
+> *"Selected work · 2018 — 2026."* — date range, two characters of meta
+> *"Twelve projects, six clients, two countries."* — three counts, no adjective
+> *"Work, indexed by year."* — five-word labels, the index is the whole site
+
+---