Unique step id; becomes the compiled GraphNode.id.
Step action type, used to select the correct node builder:
'reasoning' → gmiNode'tool_call' → toolNode'human_input' → humanNode'validation' → guardrailNodeHuman-readable description injected as the node's instructions or prompt.
Execution phase this step belongs to (governs ordering alongside anchors).
Optional toolRequired when action is 'tool_call'; the registered tool name.
Minimal plan structure produced by the current stub planner. Each step maps 1-to-1 to a
GraphNodein the compiled IR.