Workflow arguments should default to the entrypoint's inputs

# Summary

When no arguments are present on a workflow spec it should default to the entrypoint inputs.

# Use Cases

When would you use this?

I have a workflow template that is called by a cron workflow. Everything works as expected and from the CLI I can call it and pass parameters defined in the entrypoint inputs parameters. However, from the UI, users don't see the parameters unless they are defined in spec.arguments. It mean that today I need to declare them both in spec.arguments and in the entrypoint template's inputs. I think it requires more effort than it should.

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**Message from the maintainers**:

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The issue names workflow specs, spec.arguments, entrypoint inputs, the CLI, and the UI, but no repository files or tests. Start by tracing how the CLI and UI obtain workflow parameters and compare that with entrypoint input handling. Done means a workflow with no spec.arguments exposes the entrypoint inputs in the UI while preserving CLI and cron behavior, with regression coverage.

Written by the indexing model from the issue text.