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/graph.yaml

@@ -32,6 +32,7 @@ initial_state:
   verdicts: []
   gate_feedback: ''
   gate_attempts: 0
+  parse_failure: ''
 
 start: parse
 
@@ -92,8 +93,20 @@ nodes:
           type: string
           description: Repository directory named in the request (echo the prompt default when none).
       required: [findings_list, project_dir]
+    max_attempts: 2
+    state_updates:
+      parse_failure: '{{output}}'
+    fallback: parse_fault
     next: verify_each
 
+  parse_fault:
+    id: parse_fault
+    type: script
+    description: Fail-closed marker for a dead parse stage (reached only via parse's fallback) — rewrites `verdicts` to a single UNVERIFIABLE PIPELINE-FAULT entry and jumps straight to done, so the FINDING_VERIFIER_RESULTS sentinel still renders and the caller's verify lane degrades instead of dying.
+    script: scripts/parse_fault.py
+    timeout: 30
+    next: done
+
   verify_each:
     id: verify_each
     type: map
@@ -165,6 +178,7 @@ nodes:
     max_attempts: 2
     state_updates:
       verdict: '{{output}}'
+    fallback: verify_fault
     next: verdict_gate
 
   verdict_gate:
@@ -174,6 +188,13 @@ nodes:
     script: scripts/verdict_gate.py
     timeout: 30
 
+  verify_fault:
+    id: verify_fault
+    type: script
+    description: Fail-closed marker for a dead verify branch (reached only via verify_one's fallback; branch-local). Normalizes the engine's failure text in `verdict` into a schema-conformant UNVERIFIABLE verdict the map collects — a dead verifier lane never sinks the whole map or masquerades as verified.
+    script: scripts/verify_fault.py
+    timeout: 30
+
   done:
     id: done
     type: end