[CLI] Restore custom-agent service tier overrides for Standard coordinators and Fast workers

What variant of Codex are you using?

CLI

What feature would you like to see?

Please restore the ability for a custom agent role to set its own service_tier independently of the root session.

A common manager/worker setup is to use a Standard main agent for coordination, planning, delegation, and synthesis, while Fast subagents perform the actual exploration, review, and implementation work. These responsibilities have different responsiveness and usage tradeoffs. Codex should not prevent users from assigning different service tiers to them.

This setup worked before Codex CLI 0.153.0: a Standard parent could spawn custom agents whose role configuration specified service_tier = "fast". Starting with 0.153.0, #41308 deliberately made every child follow the root service tier and removed the role-level override.

The desired user-facing behavior is simple: when a custom agent role explicitly requests Fast, Codex should honor that setting even when the main agent is Standard. This lets a human define a stable policy for a role instead of requiring the main model to select a tier for each spawn.

Additional information

• #42612 contains the concrete before-and-after evidence from Codex CLI 0.152.1 and 0.153.0.
• #41308 confirms that removing role-level service-tier overrides was intentional.
• #35187 is related but distinct. It requests a per-spawn service_tier parameter that the main model can select when calling spawn_agent. This request is for a persistent setting authored by the human in the custom agent role configuration.

Start by tracing how the CLI applies the root session's service_tier when creating custom agents, using the before-and-after evidence in #42612 and the intentional change in #41308 as context. Done means a custom role explicitly set to Fast remains Fast under a Standard root, without changing the separate per-spawn request described in #35187.