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>

tests/clear_postgres_db.py

@@ -1,19 +0,0 @@
-import os
-
-from sqlalchemy import MetaData, create_engine
-
-
-def main():
-    uri = os.environ.get(
-        "MEMGPT_PGURI",
-        "postgresql+pg8000://letta:letta@localhost:8888/letta",
-    )
-
-    engine = create_engine(uri)
-    meta = MetaData()
-    meta.reflect(bind=engine)
-    meta.drop_all(bind=engine)
-
-
-if __name__ == "__main__":
-    main()