fix: keep drift watch artifacts out of source tree

[NocodeMrLi]

Summary

• keep downloaded drift reports and audit outputs under RUNNER_TEMP
• prevent validation/export tests from treating workflow runtime artifacts as source files

Verification

• python3 -m unittest tests.test_release_gate tests.test_release_quality_boundaries tests.test_phase5_freshness_tooling -q
• python3 scripts/validate_suite.py .
• workflow YAML parse check