Add the provider Checker contract and the kubernetes and GCP checks

Checker is an optional interface next to Provider: Check returns one
verdict per target and never fails as a whole. The kubernetes provider
lists at most one managed Pod cluster-wide (the verb and scope GC relies
on); the GCP provider fetches a single page of at most one managed
instance through a new Probe seam method, since the SDK treats
MaxResults as a page size and the existing AggregatedList drains every
page.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

internal/provider/kubernetes/kubernetes.go

@@ -36,6 +36,8 @@ const defaultImage = "ubuntu/squid:6.6-24.04_edge"
 type Provider struct {
 	client client.Client
 	image  string
+	// apiHost is the API server URL, shown by Check; empty in tests.
+	apiHost string
 }
 
 // New builds a kubernetes Provider from its config block. Satisfies
@@ -64,7 +66,9 @@ func NewWithTransportWrapper(_ context.Context, cfg provider.ProviderConfig, wra
 	if err != nil {
 		return nil, fmt.Errorf("kubernetes provider %q: building client: %w", cfg.Name, err)
 	}
-	return newWithClient(c, cfg), nil
+	p := newWithClient(c, cfg)
+	p.apiHost = restCfg.Host
+	return p, nil
 }
 
 // newWithClient builds a Provider around an already-constructed client,