test(ci): demote the preamble A/B to periodic (OV7) and add it to the periodic matrix

Post-Phase-3 demotion per the plan; the eval needs fetch-depth 0 (it git
shows a pre-Phase-1 sha), which only the periodic workflow provides — and
a static matrix entry so it can't silently never run.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/skill-e2e-preamble-script-ab.test.ts

@@ -33,7 +33,7 @@ import {
   carvedSkill,
 } from './helpers/auq-sdk-capture';
 
-const describeE2E = describeE2ETier('gate');
+const describeE2E = describeE2ETier('periodic');
 const runId = `preamble-ab-${process.env.EVALS_RUN_ID ?? 'local'}`;
 const ROOT = path.resolve(import.meta.dir, '..');
 const INLINE_REF = '29785978'; // last pre-Phase-1 commit (v1.69.1.0 bump)