WebFetch of claude.ai artifacts: new consent flow for reading others' artifacts while network access is off

When Claude fetches an artifact you don't own or whose ownership is unverified, the consent prompt now states plainly whether this session's network access is on or off, since that determines whether the artifact's content can enter the conversation. This replaces the older, more generic message, and the underlying check (now framed as a 'consent surface' rather than 'read is user only') runs an egress probe before granting read access.