feat: add action readiness diagnostics

[mikeholownych]

Summary

• add readiness-state and phase-report outputs to the GitHub Action for scan, upload, and SARIF phases
• document the diagnostics contract for downstream Faultline Enterprise activation and delivery troubleshooting
• add a small contract test covering the new Action outputs and diagnostics step

Test Plan

• python3 tests/test_action_contract.py
• python3 - <<'PY'
  import yaml
  with open('action.yml') as f:
  yaml.safe_load(f)
  print('action.yml ok')
  PY