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/config/profile.rs

@@ -3,6 +3,7 @@ use schemars::JsonSchema;
 use serde::Deserialize;
 use serde::Serialize;
 
+use crate::config::types::Personality;
 use crate::protocol::AskForApproval;
 use codex_protocol::config_types::ReasoningSummary;
 use codex_protocol::config_types::SandboxMode;
@@ -24,6 +25,7 @@ pub struct ConfigProfile {
     pub model_reasoning_effort: Option<ReasoningEffort>,
     pub model_reasoning_summary: Option<ReasoningSummary>,
     pub model_verbosity: Option<Verbosity>,
+    pub model_personality: Option<Personality>,
     pub chatgpt_base_url: Option<String>,
     pub experimental_instructions_file: Option<AbsolutePathBuf>,
     pub experimental_compact_prompt_file: Option<AbsolutePathBuf>,