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/docker-image.yml

@@ -1,40 +0,0 @@
-name: Docker Image CI
-
-on:
-  release:
-    types: [published]
-  workflow_dispatch:
-
-jobs:
-  build:
-    runs-on: ubuntu-latest
-
-    steps:
-    - name: Login to Docker Hub
-      uses: docker/login-action@v3
-      with:
-        username: ${{ secrets.DOCKERHUB_USERNAME }}
-        password: ${{ secrets.DOCKERHUB_TOKEN }}
-
-    - uses: actions/checkout@v3
-
-    - name: Set up QEMU
-      uses: docker/setup-qemu-action@v3
-
-    - name: Set up Docker Buildx
-      uses: docker/setup-buildx-action@v3
-
-    - name: Extract version number
-      id: extract_version
-      run: echo "CURRENT_VERSION=$(awk -F '\"' '/version =/ { print $2 }' pyproject.toml | head -n 1)" >> $GITHUB_ENV
-
-    - name: Build and push
-      uses: docker/build-push-action@v6
-      with:
-        platforms: linux/amd64,linux/arm64
-        push: true
-        tags: |
-          letta/letta:${{ env.CURRENT_VERSION }}
-          letta/letta:latest
-          memgpt/letta:${{ env.CURRENT_VERSION }}
-          memgpt/letta:latest