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/pvc_test.go

@@ -282,6 +282,64 @@ func TestPVC_SubdirCreation(t *testing.T) {
 	}
 }
 
+func TestPVC_List(t *testing.T) {
+	s := tempStore(t)
+	ctx := context.Background()
+
+	// Empty prefix — no files.
+	entries, err := s.List(ctx, "ext/pkg1/", 100)
+	if err != nil {
+		t.Fatalf("List empty: %v", err)
+	}
+	if len(entries) != 0 {
+		t.Errorf("expected 0 entries, got %d", len(entries))
+	}
+
+	// Add some files under a prefix.
+	for _, name := range []string{"a.txt", "b.png", "sub/c.pdf"} {
+		key := "ext/pkg1/" + name
+		_ = s.Put(ctx, key, bytes.NewReader([]byte("x")), 1, "text/plain")
+	}
+	// Add a file under a different prefix.
+	_ = s.Put(ctx, "ext/pkg2/other.txt", bytes.NewReader([]byte("y")), 1, "text/plain")
+
+	// List all under pkg1.
+	entries, err = s.List(ctx, "ext/pkg1/", 100)
+	if err != nil {
+		t.Fatalf("List: %v", err)
+	}
+	if len(entries) != 3 {
+		t.Errorf("expected 3 entries, got %d", len(entries))
+	}
+
+	// List with limit.
+	entries, err = s.List(ctx, "ext/pkg1/", 2)
+	if err != nil {
+		t.Fatalf("List limited: %v", err)
+	}
+	if len(entries) != 2 {
+		t.Errorf("expected 2 entries (limit), got %d", len(entries))
+	}
+
+	// List with sub-prefix.
+	entries, err = s.List(ctx, "ext/pkg1/sub/", 100)
+	if err != nil {
+		t.Fatalf("List sub: %v", err)
+	}
+	if len(entries) != 1 {
+		t.Errorf("expected 1 entry in sub/, got %d", len(entries))
+	}
+
+	// pkg2 prefix should only find 1.
+	entries, err = s.List(ctx, "ext/pkg2/", 100)
+	if err != nil {
+		t.Fatalf("List pkg2: %v", err)
+	}
+	if len(entries) != 1 {
+		t.Errorf("expected 1 entry in pkg2, got %d", len(entries))
+	}
+}
+
 func TestNewPVC_InvalidPath(t *testing.T) {
 	// Path that can't be created (nested under a file)
 	tmp := t.TempDir()