start of hooks engine (#13276)

(Experimental)

This PR adds a first MVP for hooks, with SessionStart and Stop

The core design is:

- hooks live in a dedicated engine under codex-rs/hooks
- each hook type has its own event-specific file
- hook execution is synchronous and blocks normal turn progression while
running
- matching hooks run in parallel, then their results are aggregated into
a normalized HookRunSummary

On the AppServer side, hooks are exposed as operational metadata rather
than transcript-native items:

- new live notifications: hook/started, hook/completed
- persisted/replayed hook results live on Turn.hookRuns
- we intentionally did not add hook-specific ThreadItem variants

Hooks messages are not persisted, they remain ephemeral. The context
changes they add are (they get appended to the user's prompt)

codex-rs/app-server/src/bespoke_event_handling.rs

@@ -43,6 +43,8 @@ use codex_app_server_protocol::FileChangeRequestApprovalParams;
 use codex_app_server_protocol::FileChangeRequestApprovalResponse;
 use codex_app_server_protocol::FileUpdateChange;
 use codex_app_server_protocol::GrantedPermissionProfile as V2GrantedPermissionProfile;
+use codex_app_server_protocol::HookCompletedNotification;
+use codex_app_server_protocol::HookStartedNotification;
 use codex_app_server_protocol::InterruptConversationResponse;
 use codex_app_server_protocol::ItemCompletedNotification;
 use codex_app_server_protocol::ItemStartedNotification;
@@ -1308,6 +1310,30 @@ pub(crate) async fn apply_bespoke_event_handling(
                 .send_server_notification(ServerNotification::ItemCompleted(notification))
                 .await;
         }
+        EventMsg::HookStarted(event) => {
+            if let ApiVersion::V2 = api_version {
+                let notification = HookStartedNotification {
+                    thread_id: conversation_id.to_string(),
+                    turn_id: event.turn_id,
+                    run: event.run.into(),
+                };
+                outgoing
+                    .send_server_notification(ServerNotification::HookStarted(notification))
+                    .await;
+            }
+        }
+        EventMsg::HookCompleted(event) => {
+            if let ApiVersion::V2 = api_version {
+                let notification = HookCompletedNotification {
+                    thread_id: conversation_id.to_string(),
+                    turn_id: event.turn_id,
+                    run: event.run.into(),
+                };
+                outgoing
+                    .send_server_notification(ServerNotification::HookCompleted(notification))
+                    .await;
+            }
+        }
         EventMsg::ExitedReviewMode(review_event) => {
             let review = match review_event.review_output {
                 Some(output) => render_review_output_text(&output),