Use venv inside pre-commit container instead of pip --user

Creates /tmp/venv inside the ephemeral container, installs bandit there,
and runs the security check via the venv's Python. No --user installs,
no script-location warnings, no writes outside the container's /tmp.

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

.githooks/pre-commit

@@ -20,10 +20,9 @@ docker run --rm \
   -w /repo \
   -e STAGED_FILES="$STAGED" \
   -u 1001:1001 \
-  -e HOME=/tmp \
   -e PIP_DISABLE_PIP_VERSION_CHECK=1 \
   python:3.12-slim \
-  sh -c "pip install --quiet --user --no-warn-script-location bandit && python scripts/security_check.py"
+  sh -c "python -m venv /tmp/venv && /tmp/venv/bin/pip install --quiet bandit && /tmp/venv/bin/python scripts/security_check.py"
 
 EXIT_CODE=$?