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/graph.yaml

@@ -235,7 +235,7 @@ nodes:
   default_lanes:
     id: default_lanes
     type: script
-    description: Deterministic lane-selection fallback (reached only via select_lanes' fallback) — forces code-review + adversary, plus probe when consumer_surface, when the additive LLM judgment dies. Selection degrades WIDER, never narrower; the degradation note flows into lanes_summary via build_items.
+    description: Deterministic lane-selection fallback (reached only via select_lanes' fallback) — when the additive LLM judgment dies, forces code-review + adversary, plus probe when consumer_surface, plus security on attack-surface signals (auth/deps/exec) or hardened posture, unioned with any caller-forced lanes. Selection degrades WIDER, never narrower; the degradation note flows into lanes_summary via build_items.
     script: scripts/default_lanes.py
     timeout: 30
     next: build_items