feat(auto-review) policy config (#18959)

## Summary
Allow users to customize their own auto-review policy config.

## Testing
- [x] added config_tests

codex-rs/config/src/config_toml.rs

@@ -90,6 +90,10 @@ pub struct ConfigToml {
     /// ARC.
     pub approvals_reviewer: Option<ApprovalsReviewer>,
 
+    /// Optional policy instructions for the guardian auto-reviewer.
+    #[serde(default)]
+    pub auto_review: Option<AutoReviewToml>,
+
     #[serde(default)]
     pub shell_environment_policy: ShellEnvironmentPolicyToml,
 
@@ -401,6 +405,12 @@ pub struct ConfigToml {
     pub oss_provider: Option<String>,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, Default, PartialEq, Eq, JsonSchema)]
+pub struct AutoReviewToml {
+    /// Additional policy instructions inserted into the guardian prompt.
+    pub policy: Option<String>,
+}
+
 impl From<ConfigToml> for UserSavedConfig {
     fn from(config_toml: ConfigToml) -> Self {
         let profiles = config_toml