Extend LinkedCreds data model to include Skills Array

[karenlpassmore]

OBv3 does not provide a first-class skills array, from what I can see, skills are only modeled indirectly via AlignmentObject. Our AI skill extraction and recommender workflows depend on addressing skills as discrete entities, so we need to define a skills array as a namespaced extension in our @context, while still generating OBv3 aligned properties for compatibility.