feat(TASK-029): review-gauntlet fault-only lane re-run loop

Give the review gauntlet a deterministic retry loop for lanes that FAULT
(PIPELINE-FAULT marker, missing verdict sentinel, or a nested graph's own
degraded-run wording) without ever re-running a lane that reached a real
verdict.

- graph.yaml: the four lane maps route to a new `retry_gate` script node
  (static `next: verdict_gate`; emits `_next: build_items` to retry).
  initial_state gains lane_attempts / kept_results / retry_lanes /
  review_incomplete / gauntlet_started_at. settings.timeout 24000 -> 69600
  (3 passes x 2 attempts x 11400 + 1200 margin).
- scripts/retry_gate.py (new): stashes every lane report that ran
  (completed and faulted alike) and restores reports a re-run pass
  overwrote with []; classifies with verdict_gate.py's sentinel regexes
  verbatim; counts attempts; retries while attempts < 3 and inside a
  45600 s wall-clock budget; otherwise records the lane in
  review_incomplete and falls through. Crash guard appends to
  signals_error and never retries.
- scripts/signals.py: stamps gauntlet_started_at before the git work so a
  failed diff still starts the clock.
- scripts/build_items.py: retry pass emits items only for retry_lanes
  (with attempt numbers), adds a `retried:` note to lanes_summary, resets
  retry_lanes; falsy-guarded stamp fallback; crash guard appends to
  signals_error and preserves lanes_summary.
- Tests: hermetic python3-guarded script tests for retry_gate /
  build_items / signals, graph wiring + initial_state pins, sentinel and
  LANE_KEYS cross-pins, retry-budget envelope vs settings.timeout, a
  first-pass golden payload, and an engine-level executor test proving a
  script `_next` back-edge re-fires a static fan-out and an empty map
  re-run clears its collect_into.

Plan: PLAN-review-retry section A items 1-3 and 5. Verdict/report surface
(section A items 4 and 6) is TASK-032.

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

@@ -12,6 +12,7 @@ import json
 import os
 import re
 import subprocess
+import time
 
 def load_state():
     if path := os.environ.get("GRAPH_STATE_FILE"):
@@ -37,6 +38,10 @@ def git(*args):
 
 
 out = {"project_dir": proj}
+# retry_gate measures its wall-clock budget from this stamp; it is set
+# before the git work so a failed diff still starts the clock.
+if not state.get("gauntlet_started_at"):
+    out["gauntlet_started_at"] = time.time()
 try:
     if spec in ("", "worktree"):
         names = git("diff", "--name-only", "HEAD")