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/agent/control_tests.rs

@@ -1461,6 +1461,8 @@ async fn spawn_agent_numeric_fork_from_compacted_paginated_parent_clamps_to_prov
 #[tokio::test]
 async fn spawn_agent_can_fork_parent_thread_history_with_sanitized_items() {
     let managed_fragment = "<managed_developer_instructions>\nParent developer instructions.\n</managed_developer_instructions>";
+    let persistent_fragment =
+        "<persistent_mode>\nParent developer instructions.\n</persistent_mode>";
     let harness = AgentControlHarness::new().await;
     let mut parent_config = harness.config.clone();
     let _ = parent_config.features.enable(Feature::MultiAgentV2);
@@ -1558,6 +1560,9 @@ async fn spawn_agent_can_fork_parent_thread_history_with_sanitized_items() {
                         ContentItem::InputText {
                             text: managed_fragment.to_string(),
                         },
+                        ContentItem::InputText {
+                            text: persistent_fragment.to_string(),
+                        },
                     ],
                     phase: None,
                     internal_chat_message_metadata_passthrough: Some(
@@ -1567,6 +1572,7 @@ async fn spawn_agent_can_fork_parent_thread_history_with_sanitized_items() {
                                 ContentItemKind("multi_agent.mode_instructions".to_string()),
                                 ContentItemKind("generic.developer_policy".to_string()),
                                 ContentItemKind("managed_config.developer_instructions".to_string()),
+                                ContentItemKind("persistent_mode.instructions".to_string()),
                             ]),
                             ..Default::default()
                         },
@@ -1677,6 +1683,9 @@ async fn spawn_agent_can_fork_parent_thread_history_with_sanitized_items() {
             ContentItem::InputText {
                 text: managed_fragment.to_string(),
             },
+            ContentItem::InputText {
+                text: persistent_fragment.to_string(),
+            },
         ],
         phase: None,
         internal_chat_message_metadata_passthrough: Some(
@@ -1685,6 +1694,7 @@ async fn spawn_agent_can_fork_parent_thread_history_with_sanitized_items() {
                     ContentItemKind("generic.developer_instructions".to_string()),
                     ContentItemKind("generic.developer_policy".to_string()),
                     ContentItemKind("managed_config.developer_instructions".to_string()),
+                    ContentItemKind("persistent_mode.instructions".to_string()),
                 ]),
                 ..Default::default()
             },
@@ -1769,8 +1779,9 @@ async fn spawn_agent_can_fork_parent_thread_history_with_sanitized_items() {
         "empty child developer instructions should remove parent developer instructions"
     );
     assert!(
-        history_contains_text(no_hint_history.raw_items(), managed_fragment),
-        "clearing child instructions must preserve an overlapping managed policy"
+        history_contains_text(no_hint_history.raw_items(), managed_fragment)
+            && history_contains_text(no_hint_history.raw_items(), persistent_fragment),
+        "clearing child instructions must preserve overlapping managed and persistent instructions"
     );
     assert!(
         history_contains_text(
@@ -2660,6 +2671,8 @@ async fn spawn_agent_fork_last_n_turns_drops_parent_startup_prefix_when_under_li
 
 #[tokio::test]
 async fn spawn_agent_fork_last_n_turns_strips_parent_usage_hints() {
+    let persistent_fragment =
+        "<persistent_mode>\nParent persistent instructions.\n</persistent_mode>";
     let harness = AgentControlHarness::new().await;
     let mut parent_config = harness.config.clone();
     let _ = parent_config.features.enable(Feature::MultiAgentV2);
@@ -2710,6 +2723,9 @@ async fn spawn_agent_fork_last_n_turns_strips_parent_usage_hints() {
                         ContentItem::InputText {
                             text: "Preserved bounded developer context.".to_string(),
                         },
+                        ContentItem::InputText {
+                            text: persistent_fragment.to_string(),
+                        },
                     ],
                     phase: None,
                     internal_chat_message_metadata_passthrough: None,
@@ -2772,6 +2788,10 @@ async fn spawn_agent_fork_last_n_turns_strips_parent_usage_hints() {
         !history_contains_text(history.raw_items(), "Child developer instructions."),
         "bounded fork should not inject child instructions before its canonical context rebuild"
     );
+    assert!(
+        !history_contains_text(history.raw_items(), persistent_fragment),
+        "bounded fork should remove persistent instructions before rebuilding context for the child's effort"
+    );
     assert!(
         history_contains_text(history.raw_items(), "Preserved bounded developer context."),
         "bounded fork should preserve unrelated developer fragments"