Codex fallback model instructions, unmodified.
Source: https://github.com/openai/codex/blob/2cf2a6a844f1fc2ddd489c8a67fa8bc2f59a6f3d/codex-rs/models-manager/prompt.md
Copyright OpenAI. Licensed under Apache-2.0; see LICENSE.
VELA uses this fallback for its custom Codex model catalog.
