feat: rename experimental_instructions_file to model_instructions_file (#9555)

A user who has `experimental_instructions_file` set will now see this:

<img width="888" height="660" alt="image"
src="https://github.com/user-attachments/assets/51c98312-eb9b-4881-81f1-bea6677e158d"
/>

And a `codex exec` would include this warning:

<img width="888" height="660" alt="image"
src="https://github.com/user-attachments/assets/a89f62be-1edf-4593-a75e-e0b4a762ed7d"
/>

codex-rs/core/src/codex.rs

@@ -666,6 +666,19 @@ impl Session {
                 msg: EventMsg::DeprecationNotice(DeprecationNoticeEvent { summary, details }),
             });
         }
+        if crate::config::uses_deprecated_instructions_file(&config.config_layer_stack) {
+            post_session_configured_events.push(Event {
+                id: INITIAL_SUBMIT_ID.to_owned(),
+                msg: EventMsg::DeprecationNotice(DeprecationNoticeEvent {
+                    summary: "`experimental_instructions_file` is deprecated and ignored. Use `model_instructions_file` instead."
+                        .to_string(),
+                    details: Some(
+                        "Move the setting to `model_instructions_file` in config.toml (or under a profile) to load instructions from a file."
+                            .to_string(),
+                    ),
+                }),
+            });
+        }
         maybe_push_chat_wire_api_deprecation(&config, &mut post_session_configured_events);
 
         let auth = auth_manager.auth().await;