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/state/service.rs

@@ -7,13 +7,13 @@ use crate::analytics_client::AnalyticsEventsClient;
 use crate::client::ModelClient;
 use crate::exec_policy::ExecPolicyManager;
 use crate::file_watcher::FileWatcher;
+use crate::hooks::Hooks;
 use crate::mcp_connection_manager::McpConnectionManager;
 use crate::models_manager::manager::ModelsManager;
 use crate::skills::SkillsManager;
 use crate::state_db::StateDbHandle;
 use crate::tools::sandboxing::ApprovalStore;
 use crate::unified_exec::UnifiedExecProcessManager;
-use crate::user_notification::UserNotifier;
 use codex_otel::OtelManager;
 use tokio::sync::Mutex;
 use tokio::sync::RwLock;
@@ -24,7 +24,7 @@ pub(crate) struct SessionServices {
     pub(crate) mcp_startup_cancellation_token: Mutex<CancellationToken>,
     pub(crate) unified_exec_manager: UnifiedExecProcessManager,
     pub(crate) analytics_events_client: AnalyticsEventsClient,
-    pub(crate) notifier: UserNotifier,
+    pub(crate) hooks: Hooks,
     pub(crate) rollout: Mutex<Option<RolloutRecorder>>,
     pub(crate) user_shell: Arc<crate::shell::Shell>,
     pub(crate) show_raw_agent_reasoning: bool,