Clarify minimum champions variant derivation

[KyleHerndon]

Summary

• derive legacy round-start minimum_champions from the selected variant num_agents instead of always using the first variant
• share selected-variant lookup with episode scheduling so the threshold and scheduled variant stay aligned
• update the stale Among Them validation message to refer to variant num_agents instead of Coworld token count

Validation

• No configured lint command found in pyproject.toml or repo docs
• git diff --check
• uv run pytest tests/test_commissioner_strategies.py -q
• uv run pytest tests/test_commissioner_strategies.py::test_round_start_config_infers_minimum_champions_from_selected_variant_num_agents tests/test_commissioner_strategies.py::test_round_start_adapter_uses_extracted_commissioner_api -q

Notes

• Full pytest and tests/test_commissioner_websocket.py did not complete in the local sandbox; websocket was retried with timeout 20s and timed out without test output.