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/mod.rs

@@ -11,6 +11,7 @@ use crate::config::types::Notifications;
 use crate::config::types::OtelConfig;
 use crate::config::types::OtelConfigToml;
 use crate::config::types::OtelExporterKind;
+use crate::config::types::Personality;
 use crate::config::types::SandboxWorkspaceWrite;
 use crate::config::types::ScrollInputMode;
 use crate::config::types::ShellEnvironmentPolicy;
@@ -127,6 +128,9 @@ pub struct Config {
     /// Info needed to make an API request to the model.
     pub model_provider: ModelProviderInfo,
 
+    /// Optionally specify the personality of the model
+    pub model_personality: Option<Personality>,
+
     /// Approval policy for executing commands.
     pub approval_policy: Constrained<AskForApproval>,
 
@@ -890,6 +894,10 @@ pub struct ConfigToml {
     /// Override to force-enable reasoning summaries for the configured model.
     pub model_supports_reasoning_summaries: Option<bool>,
 
+    /// EXPERIMENTAL
+    /// Optionally specify a personality for the model
+    pub model_personality: Option<Personality>,
+
     /// Base URL for requests to ChatGPT (as opposed to the OpenAI API).
     pub chatgpt_base_url: Option<String>,
 
@@ -1474,6 +1482,7 @@ impl Config {
             notify: cfg.notify,
             user_instructions,
             base_instructions,
+            model_personality: config_profile.model_personality.or(cfg.model_personality),
             developer_instructions,
             compact_prompt,
             // The config.toml omits "_mode" because it's a config file. However, "_mode"
@@ -3605,6 +3614,7 @@ model_verbosity = "high"
                 model_reasoning_summary: ReasoningSummary::Detailed,
                 model_supports_reasoning_summaries: None,
                 model_verbosity: None,
+                model_personality: None,
                 chatgpt_base_url: "https://chatgpt.com/backend-api/".to_string(),
                 base_instructions: None,
                 developer_instructions: None,
@@ -3692,6 +3702,7 @@ model_verbosity = "high"
             model_reasoning_summary: ReasoningSummary::default(),
             model_supports_reasoning_summaries: None,
             model_verbosity: None,
+            model_personality: None,
             chatgpt_base_url: "https://chatgpt.com/backend-api/".to_string(),
             base_instructions: None,
             developer_instructions: None,
@@ -3794,6 +3805,7 @@ model_verbosity = "high"
             model_reasoning_summary: ReasoningSummary::default(),
             model_supports_reasoning_summaries: None,
             model_verbosity: None,
+            model_personality: None,
             chatgpt_base_url: "https://chatgpt.com/backend-api/".to_string(),
             base_instructions: None,
             developer_instructions: None,
@@ -3882,6 +3894,7 @@ model_verbosity = "high"
             model_reasoning_summary: ReasoningSummary::Detailed,
             model_supports_reasoning_summaries: None,
             model_verbosity: Some(Verbosity::High),
+            model_personality: None,
             chatgpt_base_url: "https://chatgpt.com/backend-api/".to_string(),
             base_instructions: None,
             developer_instructions: None,