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>

examples/mcp_example.py

@@ -1,56 +0,0 @@
-from pprint import pprint
-
-from letta_client import Letta
-
-# Connect to Letta server
-client = Letta(base_url="http://localhost:8283")
-
-# Use the "everything" mcp server:
-# https://github.com/modelcontextprotocol/servers/tree/main/src/everything
-mcp_server_name = "everything"
-mcp_tool_name = "echo"
-
-# List all McpTool belonging to the "everything" mcp server.
-mcp_tools = client.tools.list_mcp_tools_by_server(
-    mcp_server_name=mcp_server_name,
-)
-
-# We can see that "echo" is one of the tools, but it's not
-# a letta tool that can be added to a client (it has no tool id).
-for tool in mcp_tools:
-    pprint(tool)
-
-# Create a Tool (with a tool id) using the server and tool names.
-mcp_tool = client.tools.add_mcp_tool(
-    mcp_server_name=mcp_server_name,
-    mcp_tool_name=mcp_tool_name
-)
-
-# Create an agent with the tool, using tool.id -- note that
-# this is the ONLY tool in the agent, you typically want to
-# also include the default tools.
-agent = client.agents.create(
-    memory_blocks=[
-        {
-            "value": "Name: Caren",
-            "label": "human"
-        }
-    ],
-    model="openai/gpt-4o-mini",
-    embedding="openai/text-embedding-3-small",
-    tool_ids=[mcp_tool.id]
-)
-print(f"Created agent id {agent.id}")
-
-# Ask the agent to call the tool.
-response = client.agents.messages.create(
-    agent_id=agent.id,
-    messages=[
-        {
-            "role": "user",
-            "content": "Hello can you echo back this input?"
-        },
-    ],
-)
-for message in response.messages:
-    print(message)