omni(meta-harness): Multi-objective search for meta-harness + Codex integration

1. Within the meta-harness path in `optimize-anything`, only a single objective metric is able to be optimized, unlike GEPA's built-in multi-objective search.
2. Additionally, the meta-harness path currently solely supports Claude Code, and there is no Codex CLI integration. Such an integration would make the method accessible on the OSS Codex harness.