refactor(cli): drop the always-empty interface_metadata wire projection

embeddedMCPMetadata only feeds interface validation now; its summary was
projected onto every schema payload as a constant-empty blob. Remove the
SchemaRegistry field, all three payload projections, the snapshot wire
field, the overview copy, the compact strip entry, and the jq policy gate.
Also delete the redundant io.Discard dead-code suppressor in
event_command.go (io has five genuine uses there).

scripts/policy/check-schema-binary.sh

@@ -101,7 +101,6 @@ jq -S '
     ]
   } +
   (if (($catalog.source // "") | type == "string" and length > 0) then {source: $catalog.source} else {} end) +
-  (if $catalog | has("interface_metadata") then {interface_metadata: $catalog.interface_metadata} else {} end) +
   (if $catalog | has("agent_metadata") then {agent_metadata: $catalog.agent_metadata} else {} end) +
   {catalog_hash: $snapshot.source_hash} +
   (if (($snapshot.surface_hash // "") | nonempty) then {surface_hash: $snapshot.surface_hash} else {} end)