test: Jest suite writes into the working tree (tracked results file and report outputs) #3482

Problem

Running the Jest suite (npx jest --config .jest.config.cjs) modifies the working tree:

• Modifies a tracked file: scripts/automation/__tests__/performance/results-phase-2b.json, written by phase-2b-validation.test.js (line 242).
• Creates untracked files: .github/reports/metrics/collection-summary-<date>.json via scripts/workflows/metrics-collection-orchestrator.cjs (exercised by metrics-collection-orchestrator.test.js), and .github/reports/changelog-metrics/<date>.json via agents/changelog-agent/includes/metricsSnapshotBuilder.cjs (default metricsDir, lines 180/226).

A developer running the suite and then git add -A or git commit -a commits test output, and CI runs are not hermetic.

Change

• Tests write only to os.tmpdir() (for example fs.mkdtempSync), or inject an output directory into the module under test.
• results-phase-2b.json: if it is a committed baseline, the test should read it and never write it; if it is output, move it to a temp dir and untrack it.
• Add a guard: after the suite, git status --porcelain must be empty. Wire this into the test workflow planned in fix(ci): no workflow runs the Jest suite, so test regressions merge green #3479.

Acceptance

• npx jest --config .jest.config.cjs && git status --porcelain prints nothing on a clean checkout.

Found while verifying #3477/#3478.