feat(personality) introduce model_personality config (#9459)

## Summary
Introduces the concept of a config model_personality. I would consider
this an MVP for testing out the feature. There are a number of
follow-ups to this PR:

- More sophisticated templating with validation
- In-product experience to manage this

## Testing
- [x] Testing locally

codex-rs/core/src/codex.rs

@@ -2974,6 +2974,7 @@ async fn run_sampling_request(
         tools: router.specs(),
         parallel_tool_calls: model_supports_parallel,
         base_instructions,
+        personality: None,
         output_schema: turn_context.final_output_json_schema.clone(),
     };