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/components/islands/SkillPackageExplorer.astro

@@ -71,7 +71,7 @@ const packageFiles: PackageFile[] = [
           class:list={[{ active: index === 0 }]}
           data-skill-file={file.id}
           role="tab"
-          aria-selected={String(index === 0)}
+          aria-selected={index === 0 ? 'true' : 'false'}
         >
           <code>{`${file.prefix}${file.label}`}</code>
           <small>{file.caption}</small>