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>

.gitattributes

@@ -1,20 +0,0 @@
-# Set the default behavior, in case people don't have core.autocrlf set.
-* text=auto
-
-# Explicitly declare text files you want to always be normalized and converted
-# to LF on checkout.
-*.py text eol=lf
-*.txt text eol=lf
-*.md text eol=lf
-*.json text eol=lf
-*.yml text eol=lf
-*.yaml text eol=lf
-
-# Declare files that will always have CRLF line endings on checkout.
-# (Only if you have specific Windows-only files)
-*.bat text eol=crlf
-
-# Denote all files that are truly binary and should not be modified.
-*.png binary
-*.jpg binary
-*.gif binary