append_subgraph is the multi-node/multi-agent generalisation of the
single-node append_node, so the two in-DAG-growth channels collapse to
one: the Reconcile planner now emits its mechanical Start/Stop as a
single-node append_subgraph rooted on the reconcile node (stamping
claim.agent on the NodeSpec, which append_node inherited implicitly).
Removes NodeOutput.append_nodes + its scheduler drain loop and
JobQueue::append_node. No behaviour change — a channel unification.
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