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/compact_remote.rs

@@ -55,6 +55,7 @@ async fn run_remote_compact_task_inner_impl(
         tools: vec![],
         parallel_tool_calls: false,
         base_instructions: sess.get_base_instructions().await,
+        personality: None,
         output_schema: None,
     };