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>

letta/cli/cli_load.py

@@ -1,16 +0,0 @@
-"""
-This file contains functions for loading data into Letta's archival storage.
-
-Data can be loaded with the following command, once a load function is defined:
-```
-letta load <data-connector-type> --name <dataset-name> [ADDITIONAL ARGS]
-```
-
-"""
-
-import typer
-
-app = typer.Typer()
-
-
-default_extensions = "txt,md,pdf"