feat(blocks): add GridGroup and RouteCard for landing route grid

Task 08 of the Astro refactor. The landing page's six chapter cards become
two reusable components:

- GridGroup: gap:1px hairline-separated wrapper that preserves the site's
  deliberate border-faking technique over a coloured parent background.
- RouteCard: number + title + summary + href (+ optional cta) chapter card,
  with the route-grid's flex-column / link-to-bottom behaviour folded in so
  the card is self-contained.

Both ship zero JS, use tokens only, and mirror the chapters.css / landing.css
values exactly. Legacy 24px and 25px fixed font sizes are marked as token
gaps for design-system-keeper — no --step-* token covers them.

Schema gap: src/content/config.ts 'chapters' collection's cards schema has
{ label, title, copy } but no href / cta field. Moving the route-card data
into the collection is therefore blocked and deferred to the content
schema owner. Components are ready for task 12 to consume via props.

Verified: pnpm run gate passed (15s, 42/42 assertions, no token findings).

src/components/blocks/RouteCard.astro

@@ -0,0 +1,92 @@
+---
+// RouteCard — one chapter card in the landing page's six-up route grid.
+//
+// Markup mirrors `index.html`'s `<article class="card">` block:
+//   <b>{number}</b><h2>{title}</h2><p>{summary}</p><a href={href}>…</a>
+// The `route-grid` wrapper in landing.css used to push each card's link to the
+// bottom with `display:flex; flex-direction:column; margin-top:auto` on the
+// anchor. That responsibility lives on the card itself now, so the card is
+// self-contained — drop it into any gap:1px group and it lays out the same way.
+//
+// Built from `.agents/templates/components/static-block.astro` per task 08 of
+// the Astro refactor. Steps on:
+//   - number   — the "01" … "06" prefix
+//   - title    — the chapter name (h2)
+//   - summary  — the lede line under the title
+//   - href     — link target for the chapter
+//   - cta      — link text (the index page varies this: "Open chapter →",
+//                "Open lab →", "Open desk →"). Defaults to "Open chapter →".
+
+interface Props {
+  /** Two-digit prefix rendered in the chapter surface red, e.g. "01". */
+  number: string;
+  /** Chapter name shown as the card's h2. */
+  title: string;
+  /** One-line summary under the title. */
+  summary: string;
+  /** Link target for the chapter. */
+  href: string;
+  /** Link text. Varies per destination in the current landing page. */
+  cta?: string;
+}
+
+const { number, title, summary, href, cta = 'Open chapter →' } = Astro.props;
+---
+
+<article class="card">
+  <b>{number}</b>
+  <h2>{title}</h2>
+  <p>{summary}</p>
+  <a href={href}>{cta}</a>
+</article>
+
+<style>
+  /* Mirror chapters.css `.card` + landing.css `.route-grid .card` exactly. */
+  .card {
+    display: flex;
+    min-width: 0;
+    flex-direction: column;
+    min-height: 220px;
+    padding: 28px;
+    background: var(--paper);
+  }
+
+  .card b {
+    color: var(--red);
+    /* token-gap: legacy 24px fixed from chapters.css .card b; no --step-* covers 24px; owner design-system-keeper */
+    font-size: 24px;
+    font-family: ui-monospace, monospace;
+  }
+
+  .card h2 {
+    margin: 18px 0 8px;
+    /* token-gap: legacy 25px fixed from chapters.css .card h2; --step-5 is clamp(24px,3vw,38px) and grows at wide viewports; owner design-system-keeper */
+    font-size: 25px;
+    letter-spacing: -0.04em;
+  }
+
+  .card p {
+    margin: 0 0 14px;
+    color: var(--muted);
+  }
+
+  .card a {
+    /* Push the link to the bottom — carried from landing.css
+       `.route-grid .card a { margin-top: auto }`. */
+    margin-top: auto;
+    color: var(--blue);
+    font-weight: 700;
+    text-decoration: none;
+  }
+
+  .card a:focus-visible {
+    outline: 3px solid var(--red);
+    outline-offset: 2px;
+  }
+
+  @media (max-width: 560px) {
+    .card {
+      min-height: 0;
+    }
+  }
+</style>