fix(TASK-019): carry PIPELINE-FAULT prefix through verdict.py's crash guard

render.py's empty-diff stub bypass is anchored on the 'PIPELINE-FAULT:'
prefix in reason/attention, but verdict.py's crash guard emitted
'verdict computation error: …' / 'verdict script error: …' without it —
on an empty diff a crashed verdict gate collapsed into the
'No changes to review.' stub and the fail-closed NEEDS-HUMAN verdict
never reached the final report.

- prefix the crash-guard reason and attention entry with PIPELINE-FAULT:
- strengthen the existing crash test to assert the prefix on both fields
- add R5 chained test: empty diff + crash-inducing state piped
  verdict.py -> render.py must render the fault, never the stub
- add crash-guard symmetry tests for parse_fault.py / aux_fault.py
  (raw non-JSON GRAPH_STATE keeps the fault marker) and a test that a
  fault survives the attention[:5] cap alongside 6 user flags

assets/agents/code-reviewer/scripts/verdict.py

@@ -196,9 +196,13 @@ except Exception as e:  # noqa: BLE001 — never crash into a silent verdict
             {
                 "verdict_out": {
                     "verdict": "NEEDS-HUMAN",
-                    "reason": f"verdict computation error: {e} — treat as needing human review",
+                    # PIPELINE-FAULT prefix is load-bearing: render.py's
+                    # empty-diff stub bypass anchors on it — without it a
+                    # crashed gate on an empty diff collapses into the
+                    # "No changes to review." stub.
+                    "reason": f"PIPELINE-FAULT: verdict computation error: {e} — human review required",
                     "counts": {}, "deferred_count": 0, "dropped_count": 0,
-                    "dropped_titles": [], "attention": [f"verdict script error: {e}"],
+                    "dropped_titles": [], "attention": [f"PIPELINE-FAULT: verdict script error: {e}"],
                     "findings_final": [],
                 }
             }