chore(config) Rename config setting to personality (#10314)

## Summary
Let's make the setting name consistent with the SlashCommand!

## Testing
- [x] Updated tests

codex-rs/exec/src/lib.rs

@@ -251,7 +251,7 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
         codex_linux_sandbox_exe,
         base_instructions: None,
         developer_instructions: None,
-        model_personality: None,
+        personality: None,
         compact_prompt: None,
         include_apply_patch_tool: None,
         show_raw_agent_reasoning: oss.then_some(true),