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>

.github/workflows/migration-test.yml

@@ -1,54 +0,0 @@
-name: Alembic Migration Tester
-on:
-  pull_request:
-    paths:
-      - '**.py'
-  workflow_dispatch:
-jobs:
-  test:
-    runs-on: ubuntu-latest
-    timeout-minutes: 15
-    services:
-      postgres:
-        image: pgvector/pgvector:pg17
-        ports:
-          - 5432:5432
-        env:
-          POSTGRES_HOST_AUTH_METHOD: trust
-          POSTGRES_DB: postgres
-          POSTGRES_USER: postgres
-        options: >-
-          --health-cmd pg_isready
-          --health-interval 10s
-          --health-timeout 5s
-          --health-retries 5
-    steps:
-      - name: Checkout
-        uses: actions/checkout@v4
-      - run: psql -h localhost -U postgres -d postgres -c 'CREATE EXTENSION vector'
-
-      - name: Set up python 3.11
-        id: setup-python
-        uses: actions/setup-python@v5
-        with:
-          python-version: 3.11
-
-      - name: Install uv
-        uses: astral-sh/setup-uv@v6
-        with:
-          enable-cache: true
-
-      - name: Install Dependencies
-        run: |
-          uv sync --all-extras
-
-      - name: Test alembic migration
-        env:
-          LETTA_PG_PORT: 5432
-          LETTA_PG_USER: postgres
-          LETTA_PG_PASSWORD: postgres
-          LETTA_PG_DB: postgres
-          LETTA_PG_HOST: localhost
-        run: |
-          uv run alembic upgrade head
-          uv run alembic check