test: full Jest run writes artefacts into the tracked tree #3498

Running the full root suite (npx jest --config .jest.config.cjs) modifies or creates files in the repository:

• modifies tracked scripts/automation/__tests__/performance/results-phase-2b.json
• creates .github/reports/changelog-metrics/
• creates .github/reports/metrics/collection-summary-<date>.json

A developer who runs the tests and then commits with a broad git add picks these up, and worktrees or CI can see a dirty tree. Tests should write to a temporary directory (os.tmpdir()) or mock the writes.

Found while verifying #3497 (2026-09-23).