Expose execution mode in hook listings (#37538)

## What changed

- Add `executionMode` to `HookMetadata` returned by `hooks/list`, with `sync` as the default for compatibility.
- Propagate each discovered hook's `sync` or `async` mode through the app-server protocol and generated schemas.
- Show the execution mode in the TUI hooks browser.

## Testing

- Cover async and default-sync modes in `hooks/list` tests and TUI snapshots.

GitOrigin-RevId: 9c9f2890798fe417face76c15847616b2dea9db4

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json

@@ -11794,6 +11794,14 @@
           "eventName": {
             "$ref": "#/definitions/v2/HookEventName"
           },
+          "executionMode": {
+            "allOf": [
+              {
+                "$ref": "#/definitions/v2/HookExecutionMode"
+              }
+            ],
+            "default": "sync"
+          },
           "handlerType": {
             "$ref": "#/definitions/v2/HookHandlerType"
           },