fix(TASK-018): harden verdict_gate fault routing and widen default_lanes fallback

Adversary finding 1: verdict_gate treated any lane report CONTAINING
'PIPELINE-FAULT:' as a lane fault, so a real review quoting the marker
silently bypassed red-finding counting and sentinel parsing. Lane-fault
detection is now precise — only a report that IS the lane_fault.py
marker (starts with it) routes to the fault rule. Adds the missing
regression tests: reds block despite MERGE-READY, NEEDS-HUMAN without
reds passes with attention, probe INCONCLUSIVE blocks with the
environment note, a gate crash fails closed, and a quoting report is
routed to the red rule, not the fault rule.

Adversary finding 2: default_lanes overwrote caller-forced lanes and
dropped the security lane the deterministic signals guaranteed. The
fallback now unions its defaults with caller-forced lanes (canonicalized
via the same aliases build_items uses) and forces security on
auth/deps/exec signals or hardened posture — selection degrades wider,
never narrower, and stays deterministic via a fixed lane order. Tests
cover forced-lane survival, alias canonicalization, unknown-name
tolerance, each security signal, and determinism.

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

@@ -3,10 +3,13 @@
 
 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.
+plus probe when the deterministic signals flagged a consumer-facing surface,
+plus security when the attack-surface signals (auth/deps/exec) or a hardened
+posture demand it, unioned with any lanes the caller forced (canonicalized
+with the same aliases build_items uses) — 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
@@ -14,6 +17,27 @@ import os
 
 MAX_DETAIL_CHARS = 500
 
+# Keep in sync with build_items.ALIASES — a caller-forced lane must
+# canonicalize the same way here as it would on the non-degraded path.
+ALIASES = {
+    "code-review": "code-review",
+    "code_review": "code-review",
+    "code-reviewer": "code-review",
+    "adversary": "adversary",
+    "adversarial": "adversary",
+    "adversarial-review": "adversary",
+    "security": "security",
+    "security-review": "security",
+    "security-reviewer": "security",
+    "probe": "probe",
+    "usage": "probe",
+    "usage-pattern": "probe",
+    "usage-pattern-testing": "probe",
+}
+
+# Stable emission order keeps the fallback deterministic.
+LANE_ORDER = ["code-review", "adversary", "security", "probe"]
+
 
 def load_state():
     if path := os.environ.get("GRAPH_STATE_FILE"):
@@ -24,9 +48,24 @@ def load_state():
 
 def main():
     state = load_state()
-    forced = ["code-review", "adversary"]
+    lanes = {"code-review", "adversary"}
     if state.get("consumer_surface"):
-        forced.append("probe")
+        lanes.add("probe")
+    # The deterministic security signals are already in state when this
+    # fallback runs; a degraded selection must never drop the security lane
+    # build_items' hard rules would have selected.
+    if (
+        state.get("touches_auth")
+        or state.get("touches_deps")
+        or state.get("touches_exec")
+        or state.get("security_posture") == "hardened"
+    ):
+        lanes.add("security")
+    # Caller-forced lanes survive degradation — union, never overwrite.
+    for lane in state.get("forced_lanes") or []:
+        if canon := ALIASES.get(str(lane).strip().lower()):
+            lanes.add(canon)
+    forced = [lane for lane in LANE_ORDER if lane in lanes]
     raw = state.get("select_lanes_failure")
     detail = raw.strip().replace("\n", " ") if isinstance(raw, str) else ""
     if len(detail) > MAX_DETAIL_CHARS: