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/client_common.rs

@@ -1,4 +1,5 @@
 use crate::client_common::tools::ToolSpec;
+use crate::config::types::Personality;
 use crate::error::Result;
 pub use codex_api::common::ResponseEvent;
 use codex_protocol::models::BaseInstructions;
@@ -35,6 +36,9 @@ pub struct Prompt {
 
     pub base_instructions: BaseInstructions,
 
+    /// Optionally specify the personality of the model.
+    pub personality: Option<Personality>,
+
     /// Optional the output schema for the model's response.
     pub output_schema: Option<Value>,
 }