feat: sub-agent injection (#12152)

This PR adds parent-thread sub-agent completion notifications and change
the prompt of the model to prevent if from being confused

codex-rs/core/src/codex_thread.rs

@@ -8,6 +8,9 @@ use crate::protocol::Event;
 use crate::protocol::Op;
 use crate::protocol::Submission;
 use codex_protocol::config_types::Personality;
+use codex_protocol::models::ContentItem;
+use codex_protocol::models::ResponseInputItem;
+use codex_protocol::models::ResponseItem;
 use codex_protocol::openai_models::ReasoningEffort;
 use codex_protocol::protocol::AskForApproval;
 use codex_protocol::protocol::SandboxPolicy;
@@ -32,7 +35,7 @@ pub struct ThreadConfigSnapshot {
 }
 
 pub struct CodexThread {
-    codex: Codex,
+    pub(crate) codex: Codex,
     rollout_path: Option<PathBuf>,
     _watch_registration: WatchRegistration,
 }
@@ -85,6 +88,33 @@ impl CodexThread {
         self.codex.session.total_token_usage().await
     }
 
+    /// Records a user-role session-prefix message without creating a new user turn boundary.
+    pub(crate) async fn inject_user_message_without_turn(&self, message: String) {
+        let pending_item = ResponseInputItem::Message {
+            role: "user".to_string(),
+            content: vec![ContentItem::InputText { text: message }],
+        };
+        let pending_items = vec![pending_item];
+        let Err(items_without_active_turn) = self
+            .codex
+            .session
+            .inject_response_items(pending_items)
+            .await
+        else {
+            return;
+        };
+
+        let turn_context = self.codex.session.new_default_turn().await;
+        let items: Vec<ResponseItem> = items_without_active_turn
+            .into_iter()
+            .map(ResponseItem::from)
+            .collect();
+        self.codex
+            .session
+            .record_conversation_items(turn_context.as_ref(), &items)
+            .await;
+    }
+
     pub fn rollout_path(&self) -> Option<PathBuf> {
         self.rollout_path.clone()
     }