feat(loop): add --run-dir isolation, heartbeat, ntfy-on-crash; scaffold start command

Closes jepa-fx-risk#11 Phase A.

- scripts/autoresearch_start.py: scaffold runs/<rq-id>/ from Council backlog leaf;
  fail-closed on non-autoresearch-ready; strips candidate_metric; writes program.md
  + run.json (provenance) + train.py copy. 19 TDD tests.
- loop.py: --run-dir flag redirects STATUS.md / metrics.json / HEARTBEAT / train.py
  into the run dir; METRICS_OUT env var passed to train subprocess so it writes
  metrics.json to the run dir; heartbeat file written each iter phase; ntfy-on-crash
  via NTFY_URL env var (best-effort).
- train.py: METRICS_OUT env var overrides metrics.json path (default unchanged).

Launch: LITELLM_KEY=xxx python loop.py --run-dir runs/rq-04

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

train.py

@@ -298,12 +298,13 @@ def main():
     phase1_r2 = float(1 - ((yte - pred_h) ** 2).sum() / ss_tot)
     print("phase1_r2   = %.4f  (n_test=%d)" % (phase1_r2, len(yte)))
 
+    _metrics_out = os.environ.get("METRICS_OUT", "metrics.json")
     json.dump({
         "val_vol_r2": val_vol_r2, "phase1_r2": phase1_r2, "n_test": len(yte),
         "knobs": {"WINDOW": WINDOW, "PATCH_LEN": PATCH_LEN,
                   "D_MODEL": D_MODEL, "DEPTH": DEPTH, "ALPHA": ALPHA,
                   "DELTA_T_MAX": DELTA_T_MAX, "EPOCHS": EPOCHS},
-    }, open("metrics.json", "w"), indent=2)
+    }, open(_metrics_out, "w"), indent=2)
     print("val_vol_r2 = %.4f  (n_test=%d, dev=%s)" % (val_vol_r2, len(yte), dev))
 
     # ── EXPORT BLOCK — do NOT edit (agent boundary) ──────────────────────────