Add Reusable Workflow Builder plugin

[workspaccode]

Add Reusable Workflow Builder (reusable-workflow-builder) plugin.
Version: 0.1.0
Description: A local Open Design scenario that turns a short workflow goal into a scoped artifact workflow with discovery, planning, generation, and review.

[lefarcen]

Hey @workspaccode, thanks for putting together the reusable-workflow-builder plugin — the scenario framing and example files make the intended flow easy to understand.

Before pool review picks this up, could you expand the PR description with Why, What users will see, Surface area, and Validation? That extra context makes plugin additions much easier to scope and verify.

[mrcfps]

@workspaccode Reviewed the new community plugin end-to-end: the manifest wiring, scenario pipeline, declared inputs, and referenced example assets all line up with the existing plugin patterns, and I didn't find any concrete correctness or maintainability issues in the changed ranges. Thanks for putting this workflow builder together — nice addition.

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