refactor: replace orchestrator with LangGraph deep-agent supervisors

- Add app/core/deep_agent.py with Home and Floating supervisor graphs
  using LangGraph create_react_agent (hierarchical pattern)
- Strip ChatAgent classes from all 4 agent files, keep @tool functions
- Rewrite output_formatter.py for event-based (token/tool_end/mutations) stream
- Update device_ws.py to use run_home_stream/run_floating_stream
- Rewrite chat.py REST route to use run_home
- Add update_core_memory tool to both supervisors
- Add langgraph>=0.3.0 to requirements.txt
- Remove orchestrator.py, execution_plan.py, agent_registry.py, plans.py
- Remove PlanAction, PlanStep, ExecutionPlan, execution_mode from schemas
- Update all affected tests to match new API
- Remove 6 deprecated test files for deleted modules
- Clean up stale docstrings referencing removed orchestrator

app/core/llm.py

@@ -1,6 +1,6 @@
 """LLM factory — centralised model instantiation via LiteLLM.
 
-Every agent and the orchestrator call ``get_llm()`` or ``get_router_llm()``
+Every agent and the deep-agent supervisors call ``get_llm()`` or ``get_router_llm()``
 instead of directly constructing a provider-specific class.  The model string
 follows the `LiteLLM model naming convention
 <https://docs.litellm.ai/docs/providers>`_: