Tell model about credentialed routes

[winston-openai]

Summary

• render credentialed route prefixes into the developer instructions bundle
• tell the model that calls to those HTTPS prefixes have stored credentials attached by the managed proxy

Validation

• just fmt
• cargo test -p codex-core (blocked by existing untouched turn_extension_data errors in core/src/tasks/mod.rs)
• just fix -p codex-core (blocked by the same existing turn_extension_data errors)

[github-actions]

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