fix(codex): pin managed agents to Terra

[HashemKhalifa]

Summary

• emit explicit model and reasoning settings in generated Codex managed-agent TOML
• pin bundled read-only TraceDecay specialists to gpt-5.6-terra with high reasoning
• extend the cross-host catalog contract to prevent future model-routing drift

Why

Generated Codex agents previously omitted model settings, allowing runtime model selection to vary with the calling session. Explicit Terra/high routing gives read-heavy specialists a stable quality and cost profile without changing their read-only sandbox or instructions.

Validation

• cargo test --test agent_suite cursor_and_codex_agents_are_generated_from_the_canonical_catalog
• cargo test --test agent_suite codex_managed_agents_export_to_user_agents_dir
• cargo fmt --check
• git diff --check

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