Add SCML arena

[Muhtasham]

Summary

• add an SCML OneShot arena backed by the upstream scml package
• include a Docker/runtime adapter for the upstream scml package that runs multiple SCML2024 OneShot simulations and maps SCML agent scores back to CodeClash players
• register the arena, add an example config, and document SCML in README and reference docs
• add unit coverage for SCML validation and result parsing

Validation

• uv run pytest -> 178 passed
• uv run pytest tests/test_integration.py -> 2 passed
• uv run pytest tests/arenas/test_scml.py -> 5 passed
• uv run ruff check codeclash/arenas/scml tests/arenas/test_scml.py codeclash/arenas/__init__.py -> passed
• local SCML runner smoke with scml==0.8.2 produced valid average-score JSON
• CodeClash SCML smoke via main.py configs/examples/SCML__dummy__r1__s2.yaml completed successfully
• GitHub Actions markdown-link-check and test are passing on the latest PR head

Notes

SCML itself remains an upstream dependency maintained outside this repository. This PR only adds the CodeClash adapter, runtime bridge, scoring integration, docs, and tests.

The arena uses the latest released scml==0.8.2 package and its exposed SCML2024OneShotWorld API. A future migration should track newer SCML world classes if upstream exposes them.