Fix scripture-plan Gospel readings labeled generic "Gospel"

Bible-plan Gospel entries in buildReadingPool never had an authored
label, so they fell back to the UI's generic "Gospel" heading. Reuse
getGospelIncipitFromCitation (already used for proper nocturn-reading
Gospels) to derive the real "A reading from the Holy Gospel
according to..." incipit from the citation when no label was
authored.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AGjUyhUZJaSjiniEmnLdak

src/hours/matins.ts

@@ -732,7 +732,12 @@ function buildReadingPool(day: LiturgicalDay, temporalId: string, date: string,
     const label = r.label ? { la: r.label.la ?? '', en: r.label.en ?? '' } : undefined;
     parts.push(
       r.isGospel
-        ? { kind: 'gospel', text: { text: r.text, status: r.status, citation: r.citation }, responsory, label }
+        ? {
+            kind: 'gospel',
+            text: { text: r.text, status: r.status, citation: r.citation },
+            responsory,
+            label: label ?? getGospelIncipitFromCitation(r.citation),
+          }
         : { kind: 'lesson', text: { text: r.text, status: r.status, citation: r.citation }, responsory, label },
     );
   }