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/skills.astro

@@ -10,14 +10,38 @@ import ChapterHero from '../components/blocks/ChapterHero.astro';
 import SkillPackageExplorer from '../components/islands/SkillPackageExplorer.astro';
 import skillsStylesheet from '../../skills/styles.css?url';
 
+// 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(`skills chapter: missing required field "${name}"`);
+  }
+  return value;
+}
+
 const base = import.meta.env.BASE_URL;
 const chapter = await getEntry('chapters', 'skills');
+if (!chapter) {
+  throw new Error('skills chapter: missing collection entry');
+}
 const lede = chapter.data.lede.en;
 const title = chapter.data.title.en;
 const eyebrow = chapter.data.eyebrow.en;
 const sections = chapter.data.sections ?? [];
 const anatomySection = sections[0];
 const createSection = sections[1];
+if (!anatomySection) {
+  throw new Error('skills chapter: missing sections[0]');
+}
+if (!createSection) {
+  throw new Error('skills chapter: missing sections[1]');
+}
+const anatomyEyebrow = requireField(anatomySection.eyebrow, 'sections[0].eyebrow');
+const anatomyCopy = requireField(anatomySection.copy, 'sections[0].copy');
+const createEyebrow = requireField(createSection.eyebrow, 'sections[1].eyebrow');
+const createSteps = requireField(createSection.steps, 'sections[1].steps');
 ---
 
 <ChapterLayout title="AI For Dummies — Skills" description={lede.replace(/<[^>]+>/g, '')}>
@@ -33,21 +57,21 @@ const createSection = sections[1];
 
   <section class="pipeline package-anatomy">
     <div>
-      <p class="eyebrow">{anatomySection.eyebrow.en}</p>
+      <p class="eyebrow">{anatomyEyebrow.en}</p>
       <h2 set:html={anatomySection.title.en} />
-      <p class="package-hint">{anatomySection.copy.en}</p>
+      <p class="package-hint">{anatomyCopy.en}</p>
     </div>
     <SkillPackageExplorer />
   </section>
 
   <section class="practice">
     <div>
-      <p class="eyebrow">{createSection.eyebrow.en}</p>
+      <p class="eyebrow">{createEyebrow.en}</p>
       <h2 set:html={createSection.title.en} />
     </div>
     <div class="steps">
       {
-        createSection.steps.map((step, index) => (
+        createSteps.map((step, index) => (
           <article>
             <b>{String(index + 1).padStart(2, '0')}</b>
             <div>