fix(chapters): narrow optional section fields in page frontmatter

The chapters schema marks section.eyebrow / panelLabel / panelCode /
steps / copy as optional, but pages /agents/, /models/, /skills/
consume them — index access without narrowing failed astro check with
ts(18048). Same for SkillPackageExplorer: aria-selected was String(bool)
which widened to plain string and failed ts(2322) against
ButtonHTMLAttributes.

Fix: guard each required field with a helper that throws a clear
message on missing data, then read .en / .map from the narrowed value.
Page rendering is unchanged — dist/ HTML for all four files is
byte-identical before and after.

No any / as any / ! / @ts-ignore. Schema and tsconfig untouched.

src/pages/agents.astro

@@ -7,8 +7,22 @@ import { getEntry } from 'astro:content';
 import ChapterLayout from '../layouts/ChapterLayout.astro';
 import ChapterHero from '../components/blocks/ChapterHero.astro';
 
+// Required-field guard. The chapters schema marks section eyebrow /
+// panelLabel / panelCode / steps / copy as optional because the schema
+// does not know which page consumes which shape. These four pages do
+// consume them — fail loudly here rather than rendering a blank section.
+function requireField<T>(value: T | undefined, name: string): T {
+  if (value === undefined) {
+    throw new Error(`agents chapter: missing required field "${name}"`);
+  }
+  return value;
+}
+
 const base = import.meta.env.BASE_URL;
 const chapter = await getEntry('chapters', 'agents');
+if (!chapter) {
+  throw new Error('agents chapter: missing collection entry');
+}
 const lede = chapter.data.lede.en;
 const title = chapter.data.title.en;
 const eyebrow = chapter.data.eyebrow.en;
@@ -16,6 +30,19 @@ const cards = chapter.data.cards ?? [];
 const sections = chapter.data.sections ?? [];
 const treeSection = sections[0];
 const handoffSection = sections[1];
+if (!treeSection) {
+  throw new Error('agents chapter: missing sections[0]');
+}
+if (!handoffSection) {
+  throw new Error('agents chapter: missing sections[1]');
+}
+// Narrow the section fields this page renders. Each `requireField` either
+// returns a non-null value or throws — TS narrows from `T | undefined` to `T`.
+const treeEyebrow = requireField(treeSection.eyebrow, 'sections[0].eyebrow');
+const treePanelLabel = requireField(treeSection.panelLabel, 'sections[0].panelLabel');
+const treePanelCode = requireField(treeSection.panelCode, 'sections[0].panelCode');
+const handoffEyebrow = requireField(handoffSection.eyebrow, 'sections[1].eyebrow');
+const handoffSteps = requireField(handoffSection.steps, 'sections[1].steps');
 ---
 
 <ChapterLayout title="AI For Dummies — Agents and trees" description={lede.replace(/<[^>]+>/g, '')}>
@@ -30,12 +57,12 @@ const handoffSection = sections[1];
 
   <section class="pipeline">
     <div>
-      <p class="eyebrow">{treeSection.eyebrow.en}</p>
+      <p class="eyebrow">{treeEyebrow.en}</p>
       <h2 set:html={treeSection.title.en} />
     </div>
     <div class="panel">
-      <strong>{treeSection.panelLabel.en}</strong>
-      <code>{treeSection.panelCode.en}</code>
+      <strong>{treePanelLabel.en}</strong>
+      <code>{treePanelCode.en}</code>
     </div>
   </section>
 
@@ -53,12 +80,12 @@ const handoffSection = sections[1];
 
   <section class="practice">
     <div>
-      <p class="eyebrow">{handoffSection.eyebrow.en}</p>
+      <p class="eyebrow">{handoffEyebrow.en}</p>
       <h2 set:html={handoffSection.title.en} />
     </div>
     <div class="steps">
       {
-        handoffSection.steps.map((step, index) => (
+        handoffSteps.map((step, index) => (
           <article>
             <b>{String(index + 1).padStart(2, '0')}</b>
             <div>