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>

letta/services/file_processor/parser/base_parser.py

@@ -1,9 +0,0 @@
-from abc import ABC, abstractmethod
-
-
-class FileParser(ABC):
-    """Abstract base class for file parser"""
-
-    @abstractmethod
-    async def extract_text(self, content: bytes, mime_type: str):
-        """Extract text from PDF content"""