refactor: migrate from create_react_agent to create_deep_agent

- Replace langgraph create_react_agent with deepagents create_deep_agent
- Sub-agents now configured as SubAgent dicts dispatched via built-in task tool
- Stream filter updated: langgraph_node 'agent' → 'model'
- Accept both AIMessage and AIMessageChunk in stream filter
- Collector only captures write mutations (insert/update/delete)
- Add deepagents>=0.4.10 to requirements.txt

requirements.txt

@@ -5,6 +5,7 @@ langchain>=0.3.0
 langchain-openai>=0.3.0
 langchain-litellm>=0.1.0
 langgraph>=0.3.0
+deepagents>=0.4.10
 litellm>=1.50.0
 pydantic>=2.10.0
 pydantic-settings>=2.7.0