fix(ci): no workflow runs the Jest suite, so test regressions merge green #3479

Problem

No active workflow runs the Jest suite. The test and checks workflows were archived on 2026-09-11 (.github/workflows/archived/2026-09-11/{testing/testing.yml,validation/checks.yml}). The planned replacement, testing-unified.yml (spec 011, T043/T044), does not exist yet.

Impact: #3477 broke 18 tests (two harnesses in workflow-consolidation-actions.test.js and changelog-unified.test.js), and every required check stayed green. The regression was caught only by a local full-suite comparison against develop.

Current develop baseline: 7 failing suites / 5 failing tests of 5,215 (tracked in #3472).

Change

• Add a PR workflow that runs Jest (.jest.config.cjs) with concurrency, a timeout, a pinned setup-node with npm cache, and npm ci --ignore-scripts.
• Gate on no new failures versus the base branch, the same approach as the changelog gate, until test: 7 failing suites / 5 failing tests on develop (2026-09-23 full-suite inventory) #3472 clears the baseline. Then gate on zero failures.
• Make it a required check on develop once it is stable.

Acceptance

• A PR that breaks a test fails the new check.
• A PR touching only docs is not slowed (paths filter or --changedSince where safe).

Relates to #3472, #2896, #3477