fix(codex): support exec optimizer backend

[TheGameVIX]

Summary

• allow codex_exec as an optimizer backend
• make --backend codex route both optimizer and target through Codex exec
• route optimizer calls through skillopt.model.codex_backend
• update backend config docs and add regression tests

Why

The Codex integration already supports running the target through codex exec, but selecting --backend codex left the optimizer on openai_chat. Users trying to run SkillOpt fully through Codex still had an API-backed optimizer path.

This keeps the existing backend split model and only adds codex_exec to the optimizer side. Claude Code remains target-only.

Validation

• python -m pytest tests/test_codex_optimizer_backend.py -q
• python -m mkdocs build --strict
• python -c "import sys; sys.argv=['train.py','--config','configs/searchqa/default.yaml','--backend','codex']; from scripts.train import parse_args, load_config; cfg=load_config(parse_args()); print(cfg['optimizer_backend'], cfg['target_backend'])" prints codex_exec codex_exec

Note: full python -m pytest -q was also attempted on Windows, but current main has unrelated path-separator assertions in ALFWorld/Sleep tests. This PR intentionally leaves those Windows-support fixes out to keep the change scoped to Codex optimizer support.

[TheGameVIX]

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