evals: default SUPERPOWERS_ROOT to parent of evals/ if unset

Adds _set_superpowers_root_default() to drill/cli.py, called at
module import after load_dotenv(). PROJECT_ROOT resolves to evals/
post-lift; its parent is the superpowers repo root, which is the
correct value for SUPERPOWERS_ROOT.

Existing env values are respected as overrides via os.environ.setdefault.

Tests:
- helper sets default when var is unset
- helper does not override when var is already set

evals/drill/cli.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import os
 import secrets
 from pathlib import Path
 
@@ -13,6 +14,22 @@ PROJECT_ROOT: Path = Path(__file__).parent.parent
 load_dotenv(PROJECT_ROOT / ".env")
 
 
+def _set_superpowers_root_default() -> None:
+    """Default SUPERPOWERS_ROOT to the parent of evals/ if not already set.
+
+    Drill historically required contributors to export SUPERPOWERS_ROOT
+    pointing at the superpowers checkout. After lifting drill into
+    superpowers/evals/, the parent of PROJECT_ROOT is always the
+    superpowers root, so we can supply this default automatically.
+
+    Existing SUPERPOWERS_ROOT environment values are respected as overrides.
+    """
+    os.environ.setdefault("SUPERPOWERS_ROOT", str(PROJECT_ROOT.parent))
+
+
+_set_superpowers_root_default()
+
+
 @click.group()
 def main() -> None:
     """Drill: Superpowers skill compliance benchmark."""