feat(core): allow guardian prompt overrides from model metadata

[charley-oai]

Summary

• add a guardian-specific developer-instructions field to model metadata
• teach core guardian prompt assembly to prefer the selected guardian model's override while keeping the JSON contract appended in code
• update affected test fixtures and add coverage for the override path

Testing

• cargo test -p codex-protocol openai_models::tests::model_info_defaults_availability_nux_to_none_when_omitted
• cargo test -p codex-core guardian_subagent_config
• cargo test -p codex-api models_client_hits_models_endpoint
• cargo test -p codex-app-server --test all get_auth_status_no_auth

[charley-oai]

@codex review this

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. More of your lovely PRs please.

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