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>

development.compose.yml

@@ -1,29 +0,0 @@
-services:
-  letta_server:
-    image: letta_server
-    hostname: letta-server
-    build:
-      context: .
-      dockerfile: Dockerfile
-      target: development
-      args:
-        - MEMGPT_ENVIRONMENT=DEVELOPMENT
-    depends_on:
-      - letta_db
-    env_file:
-      - .env
-    environment:
-      - WATCHFILES_FORCE_POLLING=true
-
-    volumes:
-      - ./letta:/letta
-      - ~/.letta/credentials:/root/.letta/credentials
-      - ./configs/server_config.yaml:/root/.letta/config
-      - ./CONTRIBUTING.md:/CONTRIBUTING.md
-      - ./tests/pytest_cache:/letta/.pytest_cache
-      - ./tests/pytest.ini:/letta/pytest.ini
-      - ./pyproject.toml:/pyproject.toml
-      - ./tests:/tests
-    ports:
-      - "8083:8083"
-      - "8283:8283"