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/config.schema.json

@@ -332,6 +332,9 @@
             "codex_git_commit": {
               "type": "boolean"
             },
+            "codex_hooks": {
+              "type": "boolean"
+            },
             "collab": {
               "type": "boolean"
             },
@@ -1815,6 +1818,9 @@
         "codex_git_commit": {
           "type": "boolean"
         },
+        "codex_hooks": {
+          "type": "boolean"
+        },
         "collab": {
           "type": "boolean"
         },