添加 hallmark 技能

skills/hallmark/references/components/h5-letter-hero.md

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+### 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>
+```