feat: deprecation warning (#5825)

<img width="955" height="311" alt="Screenshot 2025-10-28 at 14 26 25"
src="https://github.com/user-attachments/assets/99729b3d-3bc9-4503-aab3-8dc919220ab4"
/>

codex-rs/protocol/src/protocol.rs

@@ -497,6 +497,10 @@ pub enum EventMsg {
 
     ApplyPatchApprovalRequest(ApplyPatchApprovalRequestEvent),
 
+    /// Notification advising the user that something they are using has been
+    /// deprecated and should be phased out.
+    DeprecationNotice(DeprecationNoticeEvent),
+
     BackgroundEvent(BackgroundEventEvent),
 
     UndoStarted(UndoStartedEvent),
@@ -1156,6 +1160,15 @@ pub struct BackgroundEventEvent {
     pub message: String,
 }
 
+#[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
+pub struct DeprecationNoticeEvent {
+    /// Concise summary of what is deprecated.
+    pub summary: String,
+    /// Optional extra guidance, such as migration steps or rationale.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub details: Option<String>,
+}
+
 #[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
 pub struct UndoStartedEvent {
     #[serde(skip_serializing_if = "Option::is_none")]