Cyclic Graph requires explicit set_entry_point

Adding a feedback loop (reverse edge reviewer → draft_writer) to a Graph and calling build() immediately threw a ValueError.

ValueError: No entry points found - all nodes have dependencies

DAG (acyclic) graphs worked fine, but adding a cycle broke it. GraphBuilder.build() auto-detects entry points by finding nodes with no incoming edges. With cycles, all nodes have incoming edges, so detection fails.

Fix with builder.set_entry_point("draft_writer"). For cyclic graphs, you also need set_max_node_executions (prevent infinite loops) and reset_on_revisit(True) (reset state on revisit). Remember these three as a set.