#752 run_goal workflow variables not agent-scoped; two agents in one workflow collide

run_goal/init_goal use fixed workflow-variable names (agent_done/agent_turn/agent_messages/agent_llm_*/agent_tool_*). Per-run scoped, so two run_goal in the same workflow corrupt each other. Phase 0 wrapper runs one agent. Fix: optional state_key/agent_id kwarg to namespace all variables.