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>

.github/scripts/model-sweep/feature_mappings.json

@@ -1,21 +0,0 @@
-{
-  "Basic": [
-    "test_greeting_with_assistant_message",
-    "test_greeting_without_assistant_message",
-    "test_async_greeting_with_assistant_message",
-    "test_agent_loop_error",
-    "test_step_stream_agent_loop_error",
-    "test_step_streaming_greeting_with_assistant_message",
-    "test_step_streaming_greeting_without_assistant_message",
-    "test_step_streaming_tool_call",
-    "test_tool_call",
-    "test_auto_summarize"
-  ],
-  "Token Streaming": [
-    "test_token_streaming_greeting_with_assistant_message",
-    "test_token_streaming_greeting_without_assistant_message",
-    "test_token_streaming_agent_loop_error",
-    "test_token_streaming_tool_call"
-  ],
-  "Multimodal": ["test_base64_image_input", "test_url_image_input"]
-}