Add managed_execpolicy for organization-wide execution policy control

What feature would you like to see?

Currently, Codex supports admin configuration management through managed_config.toml, allowing admins to enforce standardized settings across their organization. However, there is no equivalent mechanism for managing execution policies (execpolicy) at the organization level.
We need the ability for Codex admins to setup organization-wide execution policies similar to how managed_config.toml works, ensuring consistent sandboxing, approval, and execution restrictions across all team members.

Additional information

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