fix: resolve linter issues (govet shadow, unused func)

- Rename shadowed err to mkErr in downloadImage (govet)
- Remove unused strPtr helper in test file (unused)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/channels/feishu/feishu_64.go

@@ -344,8 +344,8 @@ func (c *FeishuChannel) downloadImage(ctx context.Context, messageID, imageKey s
 	}
 
 	mediaDir := filepath.Join(os.TempDir(), "picoclaw_media")
-	if err := os.MkdirAll(mediaDir, 0o700); err != nil {
-		return "", fmt.Errorf("create media dir: %w", err)
+	if mkErr := os.MkdirAll(mediaDir, 0o700); mkErr != nil {
+		return "", fmt.Errorf("create media dir: %w", mkErr)
 	}
 
 	localPath := filepath.Join(mediaDir, uuid.New().String()[:8]+"_feishu_image")

pkg/channels/feishu/feishu_64_test.go

@@ -8,8 +8,6 @@ import (
 	larkim "github.com/larksuite/oapi-sdk-go/v3/service/im/v1"
 )
 
-func strPtr(s string) *string { return &s }
-
 func TestExtractFeishuImageKey(t *testing.T) {
 	tests := []struct {
 		name    string