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/hooks/schema/generated/stop.command.output.schema.json

@@ -0,0 +1,44 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "additionalProperties": false,
+  "definitions": {
+    "StopDecisionWire": {
+      "enum": [
+        "block"
+      ],
+      "type": "string"
+    }
+  },
+  "properties": {
+    "continue": {
+      "default": true,
+      "type": "boolean"
+    },
+    "decision": {
+      "allOf": [
+        {
+          "$ref": "#/definitions/StopDecisionWire"
+        }
+      ],
+      "default": null
+    },
+    "reason": {
+      "default": null,
+      "type": "string"
+    },
+    "stopReason": {
+      "default": null,
+      "type": "string"
+    },
+    "suppressOutput": {
+      "default": false,
+      "type": "boolean"
+    },
+    "systemMessage": {
+      "default": null,
+      "type": "string"
+    }
+  },
+  "title": "stop.command.output",
+  "type": "object"
+}