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/fern-check.yml

@@ -1,20 +0,0 @@
-name: 🌿 Fern Check
-
-on:
-  pull_request:
-    branches: [ main ]
-
-concurrency:
-  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
-  cancel-in-progress: ${{ github.ref != 'refs/heads/main' }}
-
-jobs:
-  run:
-    runs-on: [self-hosted, small]
-    steps:
-      - name: Checkout repository
-        uses: actions/checkout@v4
-
-      - name: Check API is valid
-        working-directory: apps
-        run: fern check