Add experimental remote thread store config (#18714)

Add experimental config to use remote thread store rather than local
thread store implementation in app server

codex-rs/core/config.schema.json

@@ -2299,6 +2299,10 @@
       "description": "Experimental / do not use. Replaces the synthesized realtime startup context appended to websocket session instructions. An empty string disables startup context injection entirely.",
       "type": "string"
     },
+    "experimental_thread_store_endpoint": {
+      "description": "Experimental / do not use. When set, app-server uses a remote thread store at this endpoint instead of the local filesystem/SQLite store.",
+      "type": "string"
+    },
     "experimental_use_freeform_apply_patch": {
       "type": "boolean"
     },
@@ -2957,4 +2961,4 @@
   },
   "title": "ConfigToml",
   "type": "object"
-}
+}