Migrate to checker-sdk-go v1.3.0 with standalone build tag

The SDK split the HTTP server scaffolding into the new
checker-sdk-go/checker/server subpackage. Update main.go to import
server and call server.New, and isolate the interactive form code
behind the standalone build tag so plugin/builtin builds skip
net/http and html/template entirely.

checker/definition.go

@@ -11,7 +11,7 @@ import (
 var Version = "built-in"
 
 // Definition is the package-level helper expected by the plugin
-// entrypoint and by sdk.NewServer via CheckerDefinitionProvider.
+// entrypoint and by server.New via CheckerDefinitionProvider.
 func Definition() *sdk.CheckerDefinition {
 	return (&caaProvider{}).Definition()
 }

checker/interactive.go

@@ -1,3 +1,5 @@
+//go:build standalone
+
 package checker
 
 import (