Keep default commissioner as sole baseline image

[KyleHerndon]

Summary

• remove the extra ruleset-strategy commissioner GHCR/catalog identity
• keep ghcr.io/metta-ai/commissioners-default as the default/baseline ruleset strategy image
• update ruleset commissioner build/docs to target the surviving default image

Validation

• Lint: no dedicated lint command is configured in this repo; checked local project files and workflows.
• sh -n commissioners/ruleset_strategy_commissioner/build.sh commissioners/build_image.sh
• uv run python -c "import yaml; [yaml.safe_load(open(path, encoding='utf-8')) for path in ('CATALOG.yaml', '.github/workflows/build-commissioner-images.yml')]"
• uv run pytest tests/test_commissioner_strategies.py::test_ruleset_strategy_default_config_matches_default_schedule -q
• stale-reference scan for commissioners-baseline, commissioners-ruleset-strategy, default/default_commissioner, and ruleset-strategy-commissioner is clean.

Related metta cleanup is already merged in Metta-AI/metta#15664.