Stop accepting platform-provided variant player counts

[KyleHerndon]

Summary

• Remove num_agents from commissioner VariantInfo.
• Stop deriving minimum_champions from platform variant player counts.
• Use commissioner-owned config or scheduled entries when legacy strategy code still needs an internal count.
• Update strategy tests for roster-defined scheduling.

Validation

• uv run pytest tests/test_commissioner_strategies.py -q
• uv run python -m compileall commissioners tests -q

Notes

• uv run ruff check is not available in this project environment.
• A targeted websocket integration test timed out locally before this final validation pass, so I left it out of the required PR gate.