fix: restore circleguardbench dependency and pin Python 3.12

The circleguardbench git dependency was removed in d76cb7d to fix build
failures, but this broke submission processing ("circleguardbench not
installed"). The root cause was Python 3.13 incompatibility, not the
package itself. Restore the lightweight pinned dependency (b1e0d82, no
vllm/torch) and pin python_version to 3.12 in Space config.

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

README.md

@@ -5,6 +5,7 @@ colorFrom: gray
 colorTo: indigo
 sdk: gradio
 sdk_version: 5.12.0
+python_version: "3.12"
 app_file: app.py
 pinned: true
 short_description: First benchmark testing LLM guards on safety and accuracy.

requirements.txt

@@ -7,3 +7,4 @@ python-dotenv>=1.0.0
 plotly>=5.18.0
 pydantic==2.10.6
 scikit-learn>=1.0.0
+circleguardbench @ git+https://github.com/whitecircle-ai/circle-guard-bench.git@b1e0d82