Add developer instructions for persistent mode (#41050)

## What changed

- Add bundled proactivity and follow-up guidance when `ReasoningEffort::Persistent` is selected.
- Allow model metadata to override or disable the guidance with `persistent_instructions`.
- Track the instructions as world state so effort and model changes replace or retire stale guidance without duplicating unchanged context.
- Tailor approval guidance to `send_user_message_async` availability, rebuild it for forked agents and rolled-back history, and exclude Guardian sessions.

## Testing

- Cover default, overridden, disabled, replaced, and retired instructions.
- Cover model changes, history transitions, agent forks, Guardian sessions, and asynchronous-message tool availability.

GitOrigin-RevId: 78e05d0335a70b385bd069b45edcd4ba570cef91

codex-rs/core/src/session/tests/guardian_tests.rs

@@ -1051,6 +1051,12 @@ async fn guardian_allows_unified_exec_additional_permissions_requests_past_polic
 #[tokio::test]
 async fn process_compacted_history_preserves_separate_guardian_developer_message() {
     let (session, mut turn_context) = make_session_and_context().await;
+    update_turn_settings_for_test(&mut turn_context, |settings| {
+        update_selected_settings_for_test(settings, |selected| {
+            selected.collaboration_mode.settings.reasoning_effort =
+                Some(ReasoningEffortConfig::Persistent);
+        });
+    });
     let guardian_policy = "guardian policy".to_string();
     let guardian_source =
         SessionSource::SubAgent(SubAgentSource::Other(GUARDIAN_REVIEWER_NAME.to_string()));
@@ -1121,6 +1127,12 @@ async fn process_compacted_history_preserves_separate_guardian_developer_message
             .iter()
             .any(|(message, _)| message.contains("stale developer message"))
     );
+    assert!(
+        !developer_messages
+            .iter()
+            .any(|(message, _)| message.contains("<persistent_mode>")),
+        "guardian context must not inherit persistent-mode proactivity"
+    );
     assert!(developer_messages.len() >= 2);
     assert_eq!(
         developer_messages.last(),

codex-rs/core/src/session/world_state.rs

@@ -17,6 +17,7 @@ use crate::context::world_state::ModelInstructionsState;
 use crate::context::world_state::MultiAgentModeState;
 use crate::context::world_state::MultiAgentUsageHintState;
 use crate::context::world_state::PermissionsState;
+use crate::context::world_state::PersistentModeState;
 use crate::context::world_state::PersonalityState;
 use crate::context::world_state::PluginsInstructionsState;
 use crate::context::world_state::RealtimeState;
@@ -189,6 +190,26 @@ impl Session {
                     .and_then(|messages| messages.collaboration_modes.as_ref()),
             ));
         }
+        if !crate::guardian::is_basic_session_source(&turn_context.session_source) {
+            let send_user_message_async_available =
+                !turn_context.session_source.is_non_root_agent()
+                    && step_context
+                        .settings
+                        .model_info
+                        .experimental_supported_tools
+                        .iter()
+                        .any(|tool| tool == "send_user_message_async");
+            world_state.add_section(PersistentModeState::new(
+                step_context.settings.effective_reasoning_effort().as_ref(),
+                step_context
+                    .settings
+                    .model_info
+                    .model_messages
+                    .as_ref()
+                    .and_then(|messages| messages.persistent_instructions.as_deref()),
+                send_user_message_async_available,
+            ));
+        }
         if turn_context.config.include_environment_context {
             let current_date = self
                 .services