docs: clarify claude_code_exec and codex_exec optimizer role support in config and CLI references

[RohithPariki]

Summary

Fix documentation inconsistencies around claude_code_exec and codex_exec optimizer backend support.

Changes

• Updated the backend compatibility tables in:
  • docs/reference/config.md
  • docs/guide/configuration.md
• Marked both codex_exec and claude_code_exec as supported optimizer backends.
• Updated the model.optimizer_backend parameter documentation in docs/reference/config.md to include claude_code_exec.
• Added claude_code_exec examples to docs/reference/cli.md for both training and evaluation.
• Documented the role-selection behavior of the CLI:
  • --backend claude_code_exec selects Claude Code Exec for the target role only.
  • --optimizer_backend claude_code_exec selects Claude Code Exec for both the target and optimizer roles.

Why

The current documentation does not match the implemented backend resolution behavior. claude_code_exec is supported as an optimizer backend when explicitly configured through --optimizer_backend or model.optimizer_backend, but this is not reflected consistently across the documentation.

This PR brings the backend compatibility tables, configuration reference, and CLI documentation in line with the existing implementation.