#750 run_goal tool execution is at-least-once; needs scoped sub-step for exactly-once

run_goal gates each tool observation with a workflow variable (agent_tool_<turn>_<i>). A crash between a side-effecting tool returning and its gate write can re-run it. Proper fix: scoped sub-step/child-context in DurableContext isolating checkpoint namespaces, or idempotency keys in tools.