Feat v0.8.0 files sandbox module

Bridge ObjectStore (PVC/S3) into the Starlark sandbox with extension-scoped
key namespacing. New files module with put/get/meta/list/delete/exists
builtins gated by files.read and files.write permissions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/storage/s3.go

@@ -232,6 +232,49 @@ func (s *S3Store) Exists(ctx context.Context, key string) (bool, error) {
 	return true, nil
 }
 
+// List returns objects under the given prefix, up to limit entries.
+func (s *S3Store) List(ctx context.Context, prefix string, limit int) ([]ObjectEntry, error) {
+	if prefix != "" {
+		if err := validateKey(prefix); err != nil {
+			return nil, err
+		}
+	}
+	if limit <= 0 {
+		limit = 100
+	}
+
+	fullPrefix := s.fullKey(prefix)
+	objectsCh := s.client.ListObjects(ctx, s.bucket, minio.ListObjectsOptions{
+		Prefix:    fullPrefix,
+		Recursive: true,
+	})
+
+	var entries []ObjectEntry
+	for obj := range objectsCh {
+		if obj.Err != nil {
+			return entries, fmt.Errorf("storage/s3: list %q: %w", prefix, obj.Err)
+		}
+		if len(entries) >= limit {
+			// Drain the remaining channel entries (minio requires it).
+			for range objectsCh {
+			}
+			break
+		}
+
+		// Strip the s3 prefix to recover the storage key.
+		key := strings.TrimPrefix(obj.Key, s.prefix)
+
+		ct := obj.ContentType
+		if ct == "" {
+			ct = "application/octet-stream"
+		}
+
+		entries = append(entries, ObjectEntry{Key: key, Size: obj.Size, ContentType: ct})
+	}
+
+	return entries, nil
+}
+
 // Healthy checks connectivity by performing a HeadBucket.
 func (s *S3Store) Healthy(ctx context.Context) error {
 	exists, err := s.client.BucketExists(ctx, s.bucket)