Allow overriding service_tier for sub-agents in config

What variant of Codex are you using?

cli

What feature would you like to see?

Id like to make some sub-agents to use slower tier for background tasks and make only main thread to respond faster.

Should be configurable in ~/.codex/agents/ agent.toml.

I think of

 enum Tier {
    Inherit,
    Fast,
    Flexible,
}

Additional information

_No response_