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>

tests/data/test.json

@@ -1,22 +0,0 @@
-{
-    "glossary": {
-        "title": "example glossary",
-		"GlossDiv": {
-            "title": "S",
-			"GlossList": {
-                "GlossEntry": {
-                    "ID": "SGML",
-					"SortAs": "SGML",
-					"GlossTerm": "Standard Generalized Markup Language",
-					"Acronym": "SGML",
-					"Abbrev": "ISO 8879:1986",
-					"GlossDef": {
-                        "para": "A meta-markup language, used to create markup languages such as DocBook.",
-						"GlossSeeAlso": ["GML", "XML"]
-                    },
-					"GlossSee": "markup"
-                }
-            }
-        }
-    }
-}