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/functions/dump_json.py

@@ -1,16 +0,0 @@
-import json
-
-from letta.agent import Agent
-
-
-def dump_json(self: Agent, input: str) -> str:
-    """
-    Dumps the content to JSON.
-
-    Args:
-        input (dict): dictionary object to convert to a string
-
-    Returns:
-        str: returns string version of the input
-    """
-    return json.dumps(input)