[codex] Honor role-defined spawn service tiers

[aibrahim-oai]

Why

Custom agent roles are ordinary config layers, so a role file can already express service_tier just like other config values. The spawned-agent tier path needs to preserve that effective role config and follow the same precedence pattern as model/reasoning.

What changed

• Apply an explicit spawn-time service_tier onto the child config before role application, so a role config layer can override it just like role-defined model/reasoning settings do.
• Validate the final effective child tier after the final child model is known, while still falling back to the parent tier when no child tier survives.
• Add focused integration coverage for both v1 and v2 proving role TOML loads a service tier, spawned children keep that role-configured tier, and a role tier wins over a conflicting spawn-time tier.

Validation

• just fmt
• git diff --check
• Local Rust tests not run, per repo guidance; CI should exercise the new coverage.