Read data-driven commissioner division ladder config

[KyleHerndon]

Summary

• consume division_roles / division_ladder selectors for qualifier and entry division lookup
• consume division_ladder.legacy_migrations during ruleset-strategy league migration so old tiers can be renamed or collapsed without shipping a new image
• preserve existing fallbacks for qualifiers_division_name, default_division_name, and the current Dirt/Wood migration behavior when ladder config is absent
• document that ruleset-strategy image YAML remains canonical for scheduling, while league config may provide migration compatibility data

Compatibility

This PR targets main directly and can land independently of the Metta PR.

It is safe before the Metta PR because all new reads are optional and fall back to current behavior when the new config is absent. It is also safe after the Metta PR because it starts using the added division_roles / division_ladder data when present.

Companion Metta PR: https://github.com/Metta-AI/metta/pull/16131

Validation

• uv run pytest tests/test_commissioner_strategies.py -q
• git diff --check origin/main...HEAD

No lint command is defined in pyproject.toml; the repo only declares pytest dev tooling.