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/letta-code-sync.yml

@@ -1,19 +0,0 @@
-name: Sync Code
-
-on:
-  push:
-    branches:
-      - main
-
-jobs:
-  notify:
-    runs-on: ubuntu-latest
-    if: ${{ !contains(github.event.head_commit.message, '[sync-skip]') }}
-    steps:
-      - name: Trigger repository_dispatch
-        run: |
-          curl -X POST \
-            -H "Authorization: token ${{ secrets.SYNC_PAT }}" \
-            -H "Accept: application/vnd.github.v3+json" \
-            https://api.github.com/repos/letta-ai/letta-cloud/dispatches \
-            -d '{"event_type":"oss-update"}'