feat(TASK-019): code-reviewer fail-closed review pipeline

Every llm/agent stage that can die persistently now falls back to a
machine-readable PIPELINE-FAULT sentinel instead of killing the graph:

- review_domain -> branch-local domain_fault marker (banner-prefixed
  domain_report the map collects; completeness_gate precedent)
- verify -> verdict (verifier fault renders findings unverified + a
  PIPELINE-FAULT attention line)
- parse -> parse_fault -> render (complete NEEDS-HUMAN verdict_out;
  render's no-changes stub is bypassed for fault verdicts)
- refine_groups -> cover_gate (deterministic-proposal fallback absorbs
  the failure; degradation noted in groups_note)
- aux_lanes -> aux_fault -> synthesize (aux_note fault marker; visible,
  never blocking - aux is enrichment)
- synthesize -> verdict (synth_failure marker; keyed off the failure
  prefix + non-empty diff, never findings==[], so clean zero-finding
  reviews stay MERGE-READY)

verdict.py gains unified fault accounting (R2): any PIPELINE-FAULT in
domain_reports / verifier_output / state fault keys forces NEEDS-HUMAN,
prefix-anchored so a report merely quoting the marker mid-text never
trips it; old 'PIPELINE FAULT' spelling migrated. Faults are prepended
to attention so the [:5] cap can never hide one. Dedicated *_failure
state keys capture engine failure text on every fallback+output_schema
node, keeping the bundled warning baseline at zero.

15 python3-guarded R5 tests: fault paths, quoted-marker precedence,
happy-path byte-compat regression, marker normalization, crash-path
fail-closed (verdict/cover_gate/domain_fault), verifier-fault-on-empty-
diff render, and the no-changes stub regression.

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

@@ -0,0 +1,48 @@
+#!/usr/bin/env python3
+"""Fail-closed marker for dead aux context lanes of the code-reviewer graph.
+
+Reached ONLY via aux_lanes' `fallback:` route. aux_lanes' state_updates wrote
+an "LLM node …failed: <chain>" string into `aux_failure` before routing here;
+this node records the failure as a PIPELINE-FAULT in `aux_note`, which flows
+into synthesize's "lane skips" prompt line. Aux lanes are ENRICHMENT, not
+review coverage: the degradation is made VISIBLE in the report's inputs but
+never blocks — verdict.py deliberately does NOT scan aux_note for faults.
+"""
+
+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 main():
+    state = load_state()
+    raw = state.get("aux_failure")
+    detail = raw.strip().replace("\n", " ") if isinstance(raw, str) else ""
+    if len(detail) > MAX_DETAIL_CHARS:
+        detail = detail[: MAX_DETAIL_CHARS - 1] + "…"
+    print(
+        json.dumps(
+            {"aux_note": f"PIPELINE-FAULT: aux context lanes failed after retries — {detail}"}
+        )
+    )
+
+
+try:
+    main()
+except Exception as e:  # noqa: BLE001 — the fault marker itself must never crash
+    print(
+        json.dumps(
+            {
+                "aux_note": "PIPELINE-FAULT: aux context lanes failed after retries — "
+                f"fault-marker script error: {e}"
+            }
+        )
+    )