## What changed
- Remove `ModelInfo.base_instructions` as an in-memory instruction source and use `model_messages.instructions_template` consistently for bundled, remote, fallback, and overridden model metadata.
- Preserve compatibility by promoting legacy `base_instructions` values when reading model responses and caches, and by including rendered legacy instructions when serializing `ModelsResponse` for older clients.
- Treat templates without instruction variables as literal text and retain the other model-message fields when applying instruction overrides.
This completes the consolidation proposed in https://github.com/openai/codex/pull/31302.
## Testing
- Cover legacy response and cache migration, canonical-template precedence, fallback instructions, personality rendering, overrides, and model switching.
GitOrigin-RevId: 089d986ca5e30da67db2c77a1b6a046d2cff52dc
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