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
rsync of obra/drill@013fcb8b7d into superpowers/evals/, excluding
.git/, .venv/, results/, .env/, __pycache__/, *.egg-info/,
.private-journal/.
The drill repo is unaffected by this commit; archival is a separate
manual step after this PR merges.
Source SHA recorded at evals/.drill-source-sha for divergence
detection.
