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/app-server-protocol/schema/json/ServerNotification.json

@@ -1056,6 +1056,184 @@
       },
       "type": "object"
     },
+    "HookCompletedNotification": {
+      "properties": {
+        "run": {
+          "$ref": "#/definitions/HookRunSummary"
+        },
+        "threadId": {
+          "type": "string"
+        },
+        "turnId": {
+          "type": [
+            "string",
+            "null"
+          ]
+        }
+      },
+      "required": [
+        "run",
+        "threadId"
+      ],
+      "type": "object"
+    },
+    "HookEventName": {
+      "enum": [
+        "sessionStart",
+        "stop"
+      ],
+      "type": "string"
+    },
+    "HookExecutionMode": {
+      "enum": [
+        "sync",
+        "async"
+      ],
+      "type": "string"
+    },
+    "HookHandlerType": {
+      "enum": [
+        "command",
+        "prompt",
+        "agent"
+      ],
+      "type": "string"
+    },
+    "HookOutputEntry": {
+      "properties": {
+        "kind": {
+          "$ref": "#/definitions/HookOutputEntryKind"
+        },
+        "text": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "kind",
+        "text"
+      ],
+      "type": "object"
+    },
+    "HookOutputEntryKind": {
+      "enum": [
+        "warning",
+        "stop",
+        "feedback",
+        "context",
+        "error"
+      ],
+      "type": "string"
+    },
+    "HookRunStatus": {
+      "enum": [
+        "running",
+        "completed",
+        "failed",
+        "blocked",
+        "stopped"
+      ],
+      "type": "string"
+    },
+    "HookRunSummary": {
+      "properties": {
+        "completedAt": {
+          "format": "int64",
+          "type": [
+            "integer",
+            "null"
+          ]
+        },
+        "displayOrder": {
+          "format": "int64",
+          "type": "integer"
+        },
+        "durationMs": {
+          "format": "int64",
+          "type": [
+            "integer",
+            "null"
+          ]
+        },
+        "entries": {
+          "items": {
+            "$ref": "#/definitions/HookOutputEntry"
+          },
+          "type": "array"
+        },
+        "eventName": {
+          "$ref": "#/definitions/HookEventName"
+        },
+        "executionMode": {
+          "$ref": "#/definitions/HookExecutionMode"
+        },
+        "handlerType": {
+          "$ref": "#/definitions/HookHandlerType"
+        },
+        "id": {
+          "type": "string"
+        },
+        "scope": {
+          "$ref": "#/definitions/HookScope"
+        },
+        "sourcePath": {
+          "type": "string"
+        },
+        "startedAt": {
+          "format": "int64",
+          "type": "integer"
+        },
+        "status": {
+          "$ref": "#/definitions/HookRunStatus"
+        },
+        "statusMessage": {
+          "type": [
+            "string",
+            "null"
+          ]
+        }
+      },
+      "required": [
+        "displayOrder",
+        "entries",
+        "eventName",
+        "executionMode",
+        "handlerType",
+        "id",
+        "scope",
+        "sourcePath",
+        "startedAt",
+        "status"
+      ],
+      "type": "object"
+    },
+    "HookScope": {
+      "enum": [
+        "thread",
+        "turn"
+      ],
+      "type": "string"
+    },
+    "HookStartedNotification": {
+      "properties": {
+        "run": {
+          "$ref": "#/definitions/HookRunSummary"
+        },
+        "threadId": {
+          "type": "string"
+        },
+        "turnId": {
+          "type": [
+            "string",
+            "null"
+          ]
+        }
+      },
+      "required": [
+        "run",
+        "threadId"
+      ],
+      "type": "object"
+    },
     "ItemCompletedNotification": {
       "properties": {
         "item": {
@@ -3378,6 +3556,26 @@
       "title": "Turn/startedNotification",
       "type": "object"
     },
+    {
+      "properties": {
+        "method": {
+          "enum": [
+            "hook/started"
+          ],
+          "title": "Hook/startedNotificationMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/HookStartedNotification"
+        }
+      },
+      "required": [
+        "method",
+        "params"
+      ],
+      "title": "Hook/startedNotification",
+      "type": "object"
+    },
     {
       "properties": {
         "method": {
@@ -3398,6 +3596,26 @@
       "title": "Turn/completedNotification",
       "type": "object"
     },
+    {
+      "properties": {
+        "method": {
+          "enum": [
+            "hook/completed"
+          ],
+          "title": "Hook/completedNotificationMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/HookCompletedNotification"
+        }
+      },
+      "required": [
+        "method",
+        "params"
+      ],
+      "title": "Hook/completedNotification",
+      "type": "object"
+    },
     {
       "properties": {
         "method": {