The SDK split the HTTP server scaffolding into the new
checker-sdk-go/checker/server subpackage and CheckRule.Evaluate now
returns []CheckState. Update main.go to import server and call
server.New, switch the rule and the package-level Evaluate helper to
the new slice return type, and isolate the interactive form code
behind the standalone build tag so plugin/builtin builds skip
net/http and html/template entirely.
