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/plugins/plugins.py

@@ -1,72 +0,0 @@
-import importlib
-from typing import Protocol, runtime_checkable
-
-from letta.settings import settings
-
-
-@runtime_checkable
-class SummarizerProtocol(Protocol):
-    """What a summarizer must implement"""
-
-    async def summarize(self, text: str) -> str: ...
-    def get_name(self) -> str: ...
-
-
-# Currently this supports one of each plugin type. This can be expanded in the future.
-DEFAULT_PLUGINS = {
-    "experimental_check": {
-        "protocol": None,
-        "target": "letta.plugins.defaults:is_experimental_enabled",
-    },
-    "summarizer": {
-        "protocol": SummarizerProtocol,
-        "target": "letta.services.summarizer.summarizer:Summarizer",
-    },
-}
-
-
-def get_plugin(plugin_type: str):
-    """Get a plugin instance"""
-    plugin_register = dict(DEFAULT_PLUGINS, **settings.plugin_register_dict)
-    if plugin_type in plugin_register:
-        impl_path = plugin_register[plugin_type]["target"]
-        module_path, name = impl_path.split(":")
-        module = importlib.import_module(module_path)
-        plugin = getattr(module, name)
-        if type(plugin).__name__ == "function":
-            return plugin
-        elif type(plugin).__name__ == "class":
-            if plugin_register["protocol"] and not isinstance(plugin, type(plugin_register["protocol"])):
-                raise TypeError(f"{plugin} does not implement {type(plugin_register['protocol']).__name__}")
-            return plugin()
-    raise TypeError("Unknown plugin type")
-
-
-_experimental_checker = None
-_summarizer = None
-
-
-# TODO handle coroutines
-# Convenience functions
-def get_experimental_checker():
-    global _experimental_checker
-    if _experimental_checker is None:
-        _experimental_checker = get_plugin("experimental_check")
-    return _experimental_checker
-
-
-def get_summarizer():
-    global _summarizer
-    if _summarizer is None:
-        _summarizer = get_plugin("summarizer")
-    return _summarizer
-
-
-def reset_experimental_checker():
-    global _experimental_checker
-    _experimental_checker = None
-
-
-def reset_summarizer():
-    global _summarizer
-    _summarizer = None