"Do not include into workflow" property for nodes

### Feature Idea

There were a lot of times where I need some things in each of my workflows for testing, but I don't want them to appear in final workflow attached to a result. It may be logical to add an ability to ignore some nodes when workflow will be saved. It may be done simply by a universal property for each node - a checkbox "Do not include into saved workflow".

### Existing Solutions

_No response_

### Other

_No response_

No files or tests are named. Start by tracing how node properties are defined and how workflows are serialized when saved, then determine how an opt-out flag should affect those paths; done means a node-level checkbox reliably excludes flagged nodes from saved workflows without removing them from the working graph.

Written by the indexing model from the issue text.