feat: upgrade LangGraph to 1.x and migrate deprecated APIs

- Bump langgraph from 0.4 to 1.0+, langgraph-supervisor from 0.0.12 to 0.0.30+
- Bump langchain-core, langchain-anthropic, langchain-openai to 1.x
- Add langchain>=1.0 dependency for new create_agent location
- Migrate create_react_agent -> create_agent (prompt -> system_prompt)
- Fix create_supervisor positional arg to named agents= parameter
- Replace AsyncMock checkpointer with InMemorySaver in tests (v1 type validation)
- Update version references in README, ARCHITECTURE, eng-review-plan

backend/app/graph.py

@@ -5,7 +5,7 @@ from __future__ import annotations
 import logging
 from typing import TYPE_CHECKING
 
-from langgraph.prebuilt import create_react_agent
+from langchain.agents import create_agent
 from langgraph_supervisor import create_supervisor
 
 from app.agents import get_tools_by_names
@@ -59,11 +59,11 @@ def build_agent_nodes(
             f"Permission level: {agent_config.permission}."
         )
 
-        agent_node = create_react_agent(
+        agent_node = create_agent(
             model=llm,
             tools=tools,
             name=agent_config.name,
-            prompt=system_prompt,
+            system_prompt=system_prompt,
         )
         agent_nodes.append(agent_node)
 
@@ -88,7 +88,7 @@ def build_graph(
     prompt = SUPERVISOR_PROMPT.format(agent_descriptions=agent_descriptions)
 
     workflow = create_supervisor(
-        agent_nodes,
+        agents=agent_nodes,
         model=llm,
         prompt=prompt,
         output_mode="full_history",