Change whether an asset can be shared by the guest AI

Enable/disable existing asset sharing and optionally edit asset title/description. sourceId comes from list_knowledge_sources with assetsOnly=true. Sharing readiness still requires validationStatus=ready and applicable active folder/scope; enabling does not make asset-only content answer knowledge or clear malware failures. This is separate from KB activation. Required permission: knowledge:write. Connected business only; no tenant override.