fix: keep drift detect report in runner temp

[NocodeMrLi]

Summary

• Write the detect job drift report into $RUNNER_TEMP/drift-watch instead of the checkout root.
• Upload the drift report artifact from the same temp path consumed by the audit job.
• Add a release-gate regression assertion so the workflow does not drift back to root-level artifacts.

Verification

• python3 -m unittest tests.test_release_gate.ReleaseHistoryTests.test_drift_audit_workflow_fetches_full_history tests.test_release_gate.ReleaseHistoryTests.test_drift_detect_report_stays_in_runner_temp -q
• python3 -m unittest discover -s tests -q
• python3 scripts/validate_suite.py .
• python3 scripts/check_i18n_readme_structure.py .
• python3 scripts/scan_sensitive_content.py . --format json
• git diff --check