evals: split skill-e2e-review into three per-file CI shards

Bun runs describe blocks as concurrency barriers, so the e2e-review CI
job executed its tests serially: 741s of an 860s PR critical path for
tests whose slowest member is 224s. The per-file matrix is the repo's
parallelism unit, so the split moves:

- Retro E2E + retro-base-branch  -> test/skill-e2e-retro.test.ts
- review/ship base-branch + Review Dashboard Via Attribution
                                  -> test/skill-e2e-review-attribution.test.ts
- sql-injection / enum-completeness / design-lite stay in
  test/skill-e2e-review.test.ts

One 741s job becomes three ~180-250s jobs. Locally the worst paid shard
drops from 1705s (94.7% of the 1800s kill) to under 700s. Test names,
bodies, suite strings, and eval-store collectors are unchanged, so
baselines carry over. Matrix rows added to both eval workflows
(attribution is gate-only, so no periodic row); the report job's
hardcoded runner count is gone (drift-proof).

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

.github/workflows/evals-periodic.yml

@@ -84,6 +84,11 @@ jobs:
             file: test/skill-e2e-qa-workflow.test.ts
           - name: e2e-review
             file: test/skill-e2e-review.test.ts
+          - name: e2e-retro
+            file: test/skill-e2e-retro.test.ts
+          # e2e-review-attribution is gate-only (all three of its tests are
+          # gate-tier) — deliberately absent here; an all-skip shard would just
+          # burn a container boot weekly.
           - name: e2e-workflow
             file: test/skill-e2e-workflow.test.ts
           - name: e2e-routing