The binary doubles as its own healthcheck client via the SDK's
-healthcheck flag, so the probe works in the scratch image
(no shell, no curl, no wget).
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 entirely.
