XY-568: Harden Decodex post-review PR-state readback when handoff marker already has a valid PR

[yvette-carlisle]

Summary:

• Preserve retained review handoff PR URL/head in post-review status when PR-state readback fails, with readback_warning.
• Route review-handoff PR inspection and GitHub preflight through the shared gh resolver.
• Update regression coverage and operator docs for the degraded readback path.

Validation:

• cargo make fmt
• cargo make lint-fix
• cargo make test
• python3 playbook semantic_drift_audit.py
• git diff --check