Port the risk-gate bypass fixes (puredori#11) to the shadow-week risk check (#34) #46

Problem: #34 has the same risk.yml and classifier as puredori#10, so a PR can post its own tier and odd filenames, .gitattributes -diff or test-only assertion edits can lower it.
Proposed change: port the puredori harness/risk-gate-bypasses patch (pull_request_target tier job, head fetched as data, Python -I, hardened classifier).
How to verify: the ported test_risk_tier.py passes, and a PR that edits risk.yml cannot change its own tier.