[codex] add developer instructions (#5897)

we are using developer instructions for code reviews, we need to pass
them in cli as well.

codex-rs/core/src/user_instructions.rs

@@ -40,3 +40,31 @@ impl From<UserInstructions> for ResponseItem {
         }
     }
 }
+
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
+#[serde(rename = "developer_instructions", rename_all = "snake_case")]
+pub(crate) struct DeveloperInstructions {
+    text: String,
+}
+
+impl DeveloperInstructions {
+    pub fn new<T: Into<String>>(text: T) -> Self {
+        Self { text: text.into() }
+    }
+
+    pub fn into_text(self) -> String {
+        self.text
+    }
+}
+
+impl From<DeveloperInstructions> for ResponseItem {
+    fn from(di: DeveloperInstructions) -> Self {
+        ResponseItem::Message {
+            id: None,
+            role: "developer".to_string(),
+            content: vec![ContentItem::InputText {
+                text: di.into_text(),
+            }],
+        }
+    }
+}