fix(loop): put project root on PYTHONPATH for train.py subprocess

train.py is copied into runs/<rq-id>/ by autoresearch_start.py, so when loop.py
executes it, sys.path[0] is the run dir — which has no scripts/. train.py's
frozen VaR-eval block does `from scripts.var_breach import ...`, which then
fails with ModuleNotFoundError: No module named 'scripts' on every loop run
(CI and the documented manual launch alike).

Fix in the harness, not the frozen train.py/scripts boundary: prepend the
project root (loop.py's own dir, where scripts/ lives) to the subprocess
PYTHONPATH. Verified red→green locally: the import fails without it and
resolves with it (scripts/ is an implicit namespace package, no __init__.py).

loop.py

@@ -84,6 +84,10 @@ def run_train() -> "tuple[float | None, float, str]":
     t0 = time.time()
     env = dict(os.environ)
     env["METRICS_OUT"] = str(METRICS_JSON.resolve())
+    # train.py is copied into the run dir, so sys.path[0] is that run dir — which
+    # has no scripts/. Put the project root (where loop.py + scripts/ live) on
+    # PYTHONPATH so train.py's `from scripts.var_breach import ...` resolves.
+    env["PYTHONPATH"] = str(Path(__file__).resolve().parent) + os.pathsep + env.get("PYTHONPATH", "")
     try:
         r = subprocess.run(
             [sys.executable, str(TRAIN_PY.resolve())],