refactor(core): executor and LangGraph adapter do not consume CompiledStep.executionPlan

[SingleSourceStudios]

#65 added CompiledStep.executionPlan (ExecutionPlan | null) so per-step parallel execution survives compilation. The downstream consumers do not read it yet: the executor walks DAG order flat and honours none of execution / parallel_steps / join / join_timeout, and the LangGraph adapter would need wiring to honour the new shape.

This is the consumer-side counterpart to the producer-side fix in #65, and the parallel-execution analog of #73 (which tracks the same gap for CompiledStep.verification).

Surfaced during #65 / #74.