Fix Beheading of St. John the Baptist's wrong Common category

decollation-of-st-john-baptist.yml's `common` field was set to
common-of-an-apostle -- a mistaken placeholder (he isn't an apostle) that
leaked into the Aug 29 Matins invitatory and the Lauds/Vespers office
bundle. Corrected to common-of-a-martyr, matching the file's own already-
correct `minorHoursCommon` field.

Also corrected a stale comment in the feast's own nocturn-readings file
claiming neither Nocturn 2 nor 3 has a proper responsory: Nocturn 2's own
closing responsory is genuine proper text (not a Commune reference like
Nocturn 3's), and is now authored.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WvnBMq7HGfXRyMsVLu2v4D

tests/hours/matins.test.ts

@@ -596,6 +596,16 @@ describe('resolveOrdo("matins", ...) invitatory antiphon doubling by rank', () =
     expect(openingAntiphonLatin('2026-08-24')).toContain('Regem Apostolórum Dóminum, * Veníte');
   });
 
+  it("the Beheading of St. John the Baptist (2026-08-29, common corrected from apostle to martyr) opens with the martyr Common's invitatory antiphon, not the apostle one", () => {
+    // Regression test: decollation-of-st-john-baptist.yml's `common` field
+    // used to be common-of-an-apostle (a mistaken placeholder -- he isn't
+    // an apostle), which leaked into the Matins invitatory. Corrected to
+    // common-of-a-martyr, matching the file's own already-correct
+    // `minorHoursCommon`.
+    expect(openingAntiphonLatin('2026-08-29')).toContain('Regem Mártyrum Dóminum, * Veníte');
+    expect(openingAntiphonLatin('2026-08-29')).not.toContain('Regem Apostolórum');
+  });
+
   it('a plain ferial day opens with the incipit only', () => {
     expect(openingAntiphonLatin(FERIAL_DATE)).not.toContain('Exsultémus');
   });