Retry flaky public-sync integration suite in CI

[Copilot]

test (ubuntu-latest) was failing in Actions because test/integration/pushwork.test.ts depends on public sync infrastructure and intermittently hits legacy-server Document ... is unavailable errors. This change isolates that network-sensitive suite and gives it bounded retries without weakening the rest of CI.

• What changed

  • Split the CI test invocation into:
    • the main suite, run once
    • test/integration/pushwork.test.ts, run separately with retries
  • Kept the flaky behavior scoped to the one suite that exercises public sync backends

• Why this shape

  • Prevents transient legacy sync-server failures from failing the entire Ubuntu job
  • Preserves normal signal quality for the rest of the test suite by not retrying unrelated tests

• CI shape

  - run: pnpm vitest run --exclude test/integration/pushwork.test.ts
  - run: pnpm vitest run test/integration/pushwork.test.ts --retry 2