Implement Phase 1: LangGraph backend MVP

Sets up the full backend foundation for CouncilOS:

- CouncilState TypedDict with all required fields and LangGraph reducers
- Three agent nodes: master_agent (drafts), critic_agent (scores + routes),
  writer_agent (final polish)
- LangGraph graph with cyclic rework loop: Master → Critic → (score < 8:
  back to Master | score ≥ 8: Writer → END)
- Safety valve: MAX_ITERATIONS=5 prevents infinite loops
- FastAPI app with REST endpoints (POST /api/councils/run, GET /api/councils/run/{id})
  and WebSocket endpoint (/ws/council/{run_id}) for real-time agent status events
- In-memory RunStore for Phase 1 (PostgreSQL-backed in Phase 3)
- pytest test suite: state, routing logic, critic parser, agent nodes, API endpoints
- .env.example, .gitignore, docker-compose.yml, Dockerfile

https://claude.ai/code/session_01RfMpt3TbMjZEtK3CAyP5iQ

backend/agents/writer_agent.py

@@ -0,0 +1,63 @@
+"""
+Writer Agent Node — final polishing of an approved draft.
+
+This agent receives a critic-approved draft and produces the final,
+publication-ready version with polished formatting and language.
+"""
+
+import os
+from langchain_anthropic import ChatAnthropic
+from langchain_core.messages import HumanMessage, SystemMessage
+
+from state import CouncilState
+
+
+_SYSTEM_PROMPT = """You are the Writer AI in a council of expert AIs.
+You receive a draft that has already been approved for quality by the Critic AI.
+Your job is to give it a final professional polish:
+
+- Improve sentence flow and readability
+- Ensure consistent formatting (headers, bullet points, paragraphs)
+- Fix any grammatical or stylistic issues
+- Do NOT change the factual content or overall structure
+- Preserve all key information from the draft
+
+Return only the polished document — no meta-commentary."""
+
+
+def writer_agent_node(state: CouncilState) -> dict:
+    """
+    LangGraph node function for the Writer Agent.
+
+    Receives the approved current_draft and returns a polished final version.
+
+    Args:
+        state: The current CouncilState.
+
+    Returns:
+        A dict with the final polished current_draft and updated messages.
+    """
+    llm = ChatAnthropic(
+        model="claude-3-5-sonnet-20241022",
+        api_key=os.environ.get("ANTHROPIC_API_KEY"),
+        temperature=0.4,
+        max_tokens=4096,
+    )
+
+    system_msg = SystemMessage(content=_SYSTEM_PROMPT)
+    user_msg = HumanMessage(
+        content=(
+            f"Please give a final professional polish to this approved document "
+            f"on the topic '{state['input_topic']}':\n\n"
+            f"{state['current_draft']}"
+        )
+    )
+
+    response = llm.invoke([system_msg, user_msg])
+
+    return {
+        "current_draft": response.content,
+        "messages": [system_msg, user_msg, response],
+        "active_node": "writer_agent",
+        "route_decision": "done",
+    }