kubens: use context to cancel slow-query warning timer after API returns

Replace done channel + defer with context.WithCancel so the warning
goroutine is cancelled immediately when queryNamespaces returns, not
when Run returns (which could be minutes later while user browses fzf).
Also thread the context into queryNamespaces so the k8s List call
respects it.

https://claude.ai/code/session_01XJXHq8WG22iqX8KaDb9RZz

cmd/kubens/fzf.go

@@ -16,6 +16,7 @@ package main
 
 import (
 	"bytes"
+	"context"
 	"errors"
 	"fmt"
 	"io"
@@ -54,17 +55,17 @@ func (op InteractiveSwitchOp) Run(_, stderr io.Writer) error {
 		return fmt.Errorf("cannot read current namespace: %w", err)
 	}
 
-	done := make(chan struct{})
-	defer close(done)
+	ctx, cancel := context.WithCancel(context.Background())
 	go func() {
 		select {
 		case <-time.After(3 * time.Second):
 			printer.Warning(stderr, `listing namespaces is taking long, switch to a namespace directly with "kubens -f <ns>"`)
-		case <-done:
+		case <-ctx.Done():
 		}
 	}()
 
-	ns, err := queryNamespaces(kc)
+	ns, err := queryNamespaces(ctx, kc)
+	cancel()
 	if err != nil {
 		return fmt.Errorf("could not list namespaces (is the cluster accessible?): %w", err)
 	}

cmd/kubens/list.go

@@ -53,17 +53,17 @@ func (op ListOp) Run(stdout, stderr io.Writer) error {
 		return fmt.Errorf("cannot read current namespace: %w", err)
 	}
 
-	done := make(chan struct{})
-	defer close(done)
+	ctx, cancel := context.WithCancel(context.Background())
 	go func() {
 		select {
 		case <-time.After(3 * time.Second):
 			printer.Warning(stderr, `listing namespaces is taking long, switch to a namespace directly with "kubens -f <ns>"`)
-		case <-done:
+		case <-ctx.Done():
 		}
 	}()
 
-	ns, err := queryNamespaces(kc)
+	ns, err := queryNamespaces(ctx, kc)
+	cancel()
 	if err != nil {
 		return fmt.Errorf("could not list namespaces (is the cluster accessible?): %w", err)
 	}
@@ -78,7 +78,7 @@ func (op ListOp) Run(stdout, stderr io.Writer) error {
 	return nil
 }
 
-func queryNamespaces(kc *kubeconfig.Kubeconfig) ([]string, error) {
+func queryNamespaces(ctx context.Context, kc *kubeconfig.Kubeconfig) ([]string, error) {
 	if os.Getenv("_MOCK_NAMESPACES") != "" {
 		return []string{"ns1", "ns2"}, nil
 	}
@@ -92,7 +92,7 @@ func queryNamespaces(kc *kubeconfig.Kubeconfig) ([]string, error) {
 	var next string
 	for {
 		list, err := clientset.CoreV1().Namespaces().List(
-			context.Background(),
+			ctx,
 			metav1.ListOptions{
 				Limit:    500,
 				Continue: next,