feat(cli): add ctrld diag for provisioning support

internal/controld/reachability.go

@@ -0,0 +1,33 @@
+package controld
+
+import (
+	"context"
+	"io"
+	"net/http"
+)
+
+// ProbeReachability makes one lightweight request to the ControlD API host,
+// reusing the same transport and IP-fallback logic real provisioning traffic
+// takes. Any HTTP response, even an error status, counts as reachable: this
+// checks the network path, not whether the endpoint accepts the request.
+//
+// The caller controls how long to wait via ctx; there is no timeout here
+// beyond what ctx enforces.
+func ProbeReachability(ctx context.Context, cdDev bool) error {
+	apiURL := apiURLCom
+	if cdDev {
+		apiURL = apiURLDev
+	}
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, apiURL, nil)
+	if err != nil {
+		return err
+	}
+	client := &http.Client{Transport: apiTransport(ctx, cdDev)}
+	resp, err := doWithFallback(ctx, client, req, apiServerIP(cdDev))
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+	_, _ = io.Copy(io.Discard, resp.Body)
+	return nil
+}