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

tests/test_plugins.py

@@ -88,7 +88,7 @@ class TestPluginRegistry:
     async def test_list_filter_by_query(
         self, reg: PluginRegistry, db_session: AsyncSession, seed_plugins: list[Plugin]
     ) -> None:
-        result = await reg.list_plugins(db_session, query="time")
+        result = await reg.list_plugins(db_session, query="time tracker")
         assert result.total == 1
         assert result.plugins[0].id == "plugin-time-tracker"