Add hooks implementation and wire up to notify (#9691)

This introduces a `Hooks` service. It registers hooks from config and
dispatches hook events at runtime.

N.B. The hook config is not wired up to this yet. But for legacy
reasons, we wire up `notify` from config and power it using hooks now.
Nothing about the `notify` interface has changed.

I'd start by reviewing `hooks/types.rs`

Some things to note:
  - hook names subject to change
  - no hook result yet
  - stopping semantics yet to be introduced
  - additional hooks yet to be introduced

codex-rs/core/src/codex.rs

@@ -24,6 +24,9 @@ use crate::features::FEATURES;
 use crate::features::Feature;
 use crate::features::Features;
 use crate::features::maybe_push_unstable_features_warning;
+use crate::hooks::HookEvent;
+use crate::hooks::HookEventAfterAgent;
+use crate::hooks::Hooks;
 use crate::models_manager::manager::ModelsManager;
 use crate::parse_command::parse_command;
 use crate::parse_turn_item;
@@ -35,7 +38,6 @@ use crate::stream_events_utils::last_assistant_message_from_item;
 use crate::terminal;
 use crate::truncate::TruncationPolicy;
 use crate::turn_metadata::build_turn_metadata_header;
-use crate::user_notification::UserNotifier;
 use crate::util::error_or_panic;
 use async_channel::Receiver;
 use async_channel::Sender;
@@ -201,7 +203,6 @@ use crate::tools::spec::ToolsConfig;
 use crate::tools::spec::ToolsConfigParams;
 use crate::turn_diff_tracker::TurnDiffTracker;
 use crate::unified_exec::UnifiedExecProcessManager;
-use crate::user_notification::UserNotification;
 use crate::util::backoff;
 use crate::windows_sandbox::WindowsSandboxLevelExt;
 use codex_async_utils::OrCancelExt;
@@ -1015,7 +1016,7 @@ impl Session {
                 Arc::clone(&config),
                 Arc::clone(&auth_manager),
             ),
-            notifier: UserNotifier::new(config.notify.clone()),
+            hooks: Hooks::new(config.as_ref()),
             rollout: Mutex::new(rollout_recorder),
             user_shell: Arc::new(default_shell),
             show_raw_agent_reasoning: config.show_raw_agent_reasoning,
@@ -2450,8 +2451,8 @@ impl Session {
         }
     }
 
-    pub(crate) fn notifier(&self) -> &UserNotifier {
-        &self.services.notifier
+    pub(crate) fn hooks(&self) -> &Hooks {
+        &self.services.hooks
     }
 
     pub(crate) fn user_shell(&self) -> Arc<shell::Shell> {
@@ -3776,14 +3777,21 @@ pub(crate) async fn run_turn(
 
                 if !needs_follow_up {
                     last_agent_message = sampling_request_last_agent_message;
-                    sess.notifier()
-                        .notify(&UserNotification::AgentTurnComplete {
-                            thread_id: sess.conversation_id.to_string(),
-                            turn_id: turn_context.sub_id.clone(),
-                            cwd: turn_context.cwd.display().to_string(),
-                            input_messages: sampling_request_input_messages,
-                            last_assistant_message: last_agent_message.clone(),
-                        });
+                    sess.hooks()
+                        .dispatch(crate::hooks::HookPayload {
+                            session_id: sess.conversation_id,
+                            cwd: turn_context.cwd.clone(),
+                            triggered_at: chrono::Utc::now(),
+                            hook_event: HookEvent::AfterAgent {
+                                event: HookEventAfterAgent {
+                                    thread_id: sess.conversation_id,
+                                    turn_id: turn_context.sub_id.clone(),
+                                    input_messages: sampling_request_input_messages,
+                                    last_assistant_message: last_agent_message.clone(),
+                                },
+                            },
+                        })
+                        .await;
                     break;
                 }
                 continue;
@@ -5724,7 +5732,7 @@ mod tests {
                 Arc::clone(&config),
                 Arc::clone(&auth_manager),
             ),
-            notifier: UserNotifier::new(None),
+            hooks: Hooks::new(&config),
             rollout: Mutex::new(None),
             user_shell: Arc::new(default_user_shell()),
             show_raw_agent_reasoning: config.show_raw_agent_reasoning,
@@ -5854,7 +5862,7 @@ mod tests {
                 Arc::clone(&config),
                 Arc::clone(&auth_manager),
             ),
-            notifier: UserNotifier::new(None),
+            hooks: Hooks::new(&config),
             rollout: Mutex::new(None),
             user_shell: Arc::new(default_user_shell()),
             show_raw_agent_reasoning: config.show_raw_agent_reasoning,