fix: make monthly audit entrypoint import service package

[Pigbibi]

Summary

• add the repository root to sys.path before the monthly audit entrypoint imports service.model_router
• add a subprocess regression test matching the GitHub Actions invocation (python scripts/run_monthly_codex_audit.py without PYTHONPATH)

Production evidence

• AIAuditBridge run 30692076738 failed on Ubuntu with ModuleNotFoundError: No module named 'service'
• the same command reproduced locally before the fix

Validation

• regression test was RED before the fix and passes after it
• python3 -m unittest tests.test_run_monthly_codex_audit (115 tests, 1 skipped)
• python3 -m ruff check scripts/run_monthly_codex_audit.py tests/test_run_monthly_codex_audit.py