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/manually_clear_old_issues.yml

@@ -1,25 +0,0 @@
-name: Clear Old Issues
-on:
-  workflow_dispatch:
-
-jobs:
-  cleanup-old-issues:
-    runs-on: ubuntu-latest
-    permissions:
-      issues: write
-      pull-requests: write
-    steps:
-      - uses: actions/stale@v5
-        with:
-          days-before-issue-stale: 60
-          days-before-issue-close: 0
-          stale-issue-label: "auto-closed"
-          stale-issue-message: ""
-          close-issue-message: "This issue has been automatically closed due to 60 days of inactivity."
-          days-before-pr-stale: -1
-          days-before-pr-close: -1
-          exempt-issue-labels: ""
-          only-issue-labels: ""
-          remove-stale-when-updated: true
-          operations-per-run: 1000
-          repo-token: ${{ secrets.GITHUB_TOKEN }}