feat(TASK-018): review-gauntlet lane degradation + verification passthrough

- lane_fault.py + fallback on all four run_* lane nodes: an agent-node
  failure is normalized into a PIPELINE-FAULT lane result instead of
  killing the graph
- verdict_gate.py: PIPELINE-FAULT lane results block distinctly from
  SKIPPED and missing-sentinel; a PIPELINE-FAULT in signals_error blocks
  at the gauntlet level
- build_items.py: restructured into main() + fail-closed guard (crash
  emits empty lanes + PIPELINE-FAULT) and folds lanes_degraded into
  lanes_summary
- parse: state_updates/fallback -> parse_fault.py, which records the
  fault in signals_error and jumps to the verdict gate
- select_lanes: state_updates/fallback -> default_lanes.py, a
  deterministic wider-never-narrower lane fallback
- verification_commands threaded through parse into run_adversary's
  prompt (declared-only, never invented)
- run_probe prompt gains the aborted-attempt reconcile line
- 8 gauntlet_* R5 tests in src/config/request_context.rs (script harness
  + graph interpolation)

assets/agents/review-gauntlet/scripts/default_lanes.py

@@ -0,0 +1,51 @@
+#!/usr/bin/env python3
+"""Deterministic lane-selection fallback for the review gauntlet.
+
+Reached ONLY via select_lanes' `fallback:` route, when the additive LLM
+judgment dies after retries. Emits `forced_lanes` — code-review + adversary,
+plus probe when the deterministic signals flagged a consumer-facing
+surface — which build_items honors exactly, so selection degrades WIDER,
+never narrower. The degradation note goes into `lanes_degraded`; build_items
+folds it into lanes_summary so the final report names the degradation.
+"""
+
+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()
+    forced = ["code-review", "adversary"]
+    if state.get("consumer_surface"):
+        forced.append("probe")
+    raw = state.get("select_lanes_failure")
+    detail = raw.strip().replace("\n", " ") if isinstance(raw, str) else ""
+    if len(detail) > MAX_DETAIL_CHARS:
+        detail = detail[: MAX_DETAIL_CHARS - 1] + "…"
+    note = "lane selection degraded to deterministic defaults"
+    if detail:
+        note += f" — {detail}"
+    print(json.dumps({"forced_lanes": forced, "lanes_degraded": note}))
+
+
+try:
+    main()
+except Exception as e:  # noqa: BLE001 — the fallback itself must never crash
+    print(
+        json.dumps(
+            {
+                "forced_lanes": ["code-review", "adversary"],
+                "lanes_degraded": "lane selection degraded to deterministic "
+                f"defaults (fallback script error: {e})",
+            }
+        )
+    )