feat(TASK-020): finding-verifier + step-runner fail-closed hardening

finding-verifier: parse gains max_attempts 2 + parse_failure capture +
fallback to a new parse_fault marker that rewrites verdicts to a single
UNVERIFIABLE PIPELINE-FAULT entry and continues to done (sentinel
preserved — the caller's verify lane degrades instead of dying);
verify_one gains a branch-local verify_fault marker normalizing the
engine failure text into a schema-conformant UNVERIFIABLE verdict.

step-runner: implement gains fallback end_failure (no retry — reruns
against a mutated tree are unsafe; STEP_FAILED renders the failure via
coder_result); independent_review gains fallback write_handoff with the
handoff instructions and route_review treating an "Agent node failed:"
review_report as review-did-not-run (fail-visible, guard ahead of the
🔴 grep); orient/edge_case_sweep/write_handoff capture their failure
text via state_updates, shrinking the bundled fallback-capture warning
baseline back to zero.

R5 tests: fault-marker normalization + crash paths (raw non-JSON
GRAPH_STATE), route_review.sh fault/critical/clean routing via bash+jq,
and structural wiring tests pinning fallbacks, sentinels, and
end_failure's initial_state-backed template keys.

assets/agents/finding-verifier/scripts/parse_fault.py

@@ -0,0 +1,57 @@
+#!/usr/bin/env python3
+"""Fail-closed marker for a dead parse stage of the finding-verifier graph.
+
+Reached ONLY via parse's `fallback:` route. parse's state_updates wrote an
+"LLM node …failed: <chain>" string into `parse_failure` before routing here;
+this node rewrites `verdicts` to a single schema-conformant UNVERIFIABLE
+entry naming the fault and the graph jumps straight to `done` — the
+FINDING_VERIFIER_RESULTS sentinel still renders, so the caller (e.g.
+code-reviewer's verify lane) degrades instead of dying. Findings that were
+never extracted cannot be verified; the caller's "UNVERIFIABLE = keep the
+finding, marked unverified" semantics do the right thing with the entry.
+"""
+
+import json
+import os
+
+MAX_DETAIL_CHARS = 500
+
+
+def load_state():
+    if path := os.environ.get("GRAPH_STATE_FILE"):
+        with open(path) as f:
+            return json.load(f)
+    return json.loads(os.environ.get("GRAPH_STATE", "{}"))
+
+
+def fault_verdicts(detail):
+    return {
+        "verdicts": [
+            {
+                "id": "pipeline-fault",
+                "verdict": "UNVERIFIABLE",
+                "evidence": "",
+                "note": (
+                    "PIPELINE-FAULT: parse failed — findings could not be extracted "
+                    f"from the verification request; no finding was verified: {detail}"
+                ),
+            }
+        ]
+    }
+
+
+def main():
+    state = load_state()
+    raw = state.get("parse_failure")
+    detail = raw.strip().replace("\n", " ") if isinstance(raw, str) else ""
+    if not detail.startswith("LLM node"):
+        detail = "the parse stage died without recording a failure detail"
+    if len(detail) > MAX_DETAIL_CHARS:
+        detail = detail[: MAX_DETAIL_CHARS - 1] + "…"
+    print(json.dumps(fault_verdicts(detail)))
+
+
+try:
+    main()
+except Exception as e:  # noqa: BLE001 — the fault marker itself must never crash
+    print(json.dumps(fault_verdicts(f"fault-marker script error: {e}")))