coreapi commands: collapse create/add closures to match repo create

/simplify pass on the spec-refresh diff. The .Response-drop left five create/add closures in a verbose 'sc, err := …; if err…; return sc, nil' form, while the sibling repo create already returns the call directly. The generated methods now return (*T, error) and runCoreJSON takes (any, error), so forward the call: return c.CreateX(ctx, body, …). Also drop the leftover 'created := sc' alias in mirror create — name the result 'created' at the call site.

No behavior change; build/lint/test green.