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/core/src/features.rs

@@ -95,6 +95,8 @@ pub enum Feature {
     ApplyPatchFreeform,
     /// Allow requesting additional filesystem permissions while staying sandboxed.
     RequestPermissions,
+    /// Enable Claude-style lifecycle hooks loaded from hooks.json files.
+    CodexHooks,
     /// Expose the built-in request_permissions tool.
     RequestPermissionsTool,
     /// Allow the model to request web searches that fetch live content.
@@ -591,6 +593,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::UnderDevelopment,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::CodexHooks,
+        key: "codex_hooks",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::RequestPermissionsTool,
         key: "request_permissions_tool",