feat(harness): add local preview capability preflight

Add local preview capability discovery and preflight. Report GUI availability, supported URL schemes, filesystem visibility and browser reachability with verified, unsupported or unknown results. Do not infer shared localhost or sandbox isolation from a failed connection alone. Return actionable Artifact alternatives. Acceptance: covered supported, unsupported and unknown paths; failure guidance explains the boundary; readiness claims require evidence. Relates to #5213.