Fix DAG clone mutations affecting original instances

The previous Arc<RwLock<Node<T>>> implementation allowed modifications to
DAG clones to mutate the original instances through shared references,
causing duplicate tasks to appear in workflows.

Replace with immutable Arc<Node<T>> design where cloning creates truly
independent DAG instances. Node values use Arc<T> for efficient sharing
without requiring T: Clone on all operations.

It does introduce the trade-off that the DAG needs to be re-built on
concatenation in order to replace the existing tail node, however, as
long as plans don't get too large, this should not introduce too much
overhead

Change-type: patch

Author: pipex