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/docs/node/package.json

@@ -1,18 +0,0 @@
-{
-  "name": "@letta-ai/core",
-  "version": "0.1.0",
-  "private": true,
-  "type": "module",
-  "scripts": {
-    "example": "node --no-warnings --import 'data:text/javascript,import { register } from \"node:module\"; import { pathToFileURL } from \"node:url\"; register(\"ts-node/esm\", pathToFileURL(\"./\"));' example.ts",
-    "build": "tsc"
-  },
-  "dependencies": {
-    "@letta-ai/letta-client": "^0.1.131"
-  },
-  "devDependencies": {
-    "@types/node": "^22.12.0",
-    "ts-node": "^10.9.2",
-    "typescript": "^5.7.3"
-  }
-}