chore: officially migrate to submodule (#4502)

* remove apps/core and apps/fern

* fix precommit

* add submodule updates in workflows

* submodule

* remove core tests

* update core revision

* Add submodules: true to all GitHub workflows

- Ensure all workflows can access git submodules
- Add submodules support to deployment, test, and CI workflows
- Fix YAML syntax issues in workflow files

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* remove core-lint

* upgrade core with latest main of oss

---------

Co-authored-by: Claude <noreply@anthropic.com>

examples/tutorials/memgpt-admin-client.ipynb

@@ -1,50 +0,0 @@
-{
- "cells": [
-  {
-   "cell_type": "code",
-   "execution_count": null,
-   "id": "fb13c7bc-fbb4-4ccd-897c-08995db258e8",
-   "metadata": {},
-   "outputs": [],
-   "source": [
-    "from letta import Admin \n",
-    "\n",
-    "base_url=\"letta.localhost\"\n",
-    "token=\"lettaadmin\" \n",
-    "\n",
-    "admin_client = Admin(base_url=base_url, token=\"lettaadmin\")"
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": null,
-   "id": "984b8249-a3f7-40d1-9691-4d128f9a90ff",
-   "metadata": {},
-   "outputs": [],
-   "source": [
-    "user = admin_client.create_user()"
-   ]
-  }
- ],
- "metadata": {
-  "kernelspec": {
-   "display_name": "letta",
-   "language": "python",
-   "name": "letta"
-  },
-  "language_info": {
-   "codemirror_mode": {
-    "name": "ipython",
-    "version": 3
-   },
-   "file_extension": ".py",
-   "mimetype": "text/x-python",
-   "name": "python",
-   "nbconvert_exporter": "python",
-   "pygments_lexer": "ipython3",
-   "version": "3.12.2"
-  }
- },
- "nbformat": 4,
- "nbformat_minor": 5
-}