Fix ResponseValidationError: change response field to content

FastAPI validation was failing because stage1 results used 'response'
field but Pydantic expected 'content'. Changed field name in:
- stage1_collect_responses: returns content instead of response
- stage2_collect_rankings: references result['content']
- stage3_synthesize_final: references result['content']
- MCP server: reads resp.get("content", "") for content_blocks

mcp_server/server.py

@@ -145,7 +145,7 @@ async def council_query(
             "type": "council_response",
             "model": model_id,
             "model_display_name": get_display_name(model_id),
-            "response": resp.get("response", ""),
+            "response": resp.get("content", ""),
             "stage": 1
         })