Implement idempotent cleanup behavior

internal/fileops/cleanup.go

@@ -13,6 +13,9 @@ import (
 func RemoveAllUnderRoot(rootPath, target string) error {
 	root, targetName, err := openCleanupRoot(rootPath, target)
 	if err != nil {
+		if errors.Is(err, os.ErrNotExist) {
+			return nil
+		}
 		return err
 	}
 	defer func() { _ = root.Close() }()

internal/fileops/cleanup_test.go

@@ -34,6 +34,14 @@ func TestRemoveAllUnderRootRemovesNestedTreeIdempotently(t *testing.T) {
 	}
 }
 
+func TestCleanupRemoveAllUnderRootAcceptsMissingRoot(t *testing.T) {
+	root := filepath.Join(t.TempDir(), "absent")
+	target := filepath.Join(root, "runs", "run-1")
+	if err := RemoveAllUnderRoot(root, target); err != nil {
+		t.Fatalf("RemoveAllUnderRoot() with missing root error = %v", err)
+	}
+}
+
 func TestRemoveAllUnderRootRejectsUnsafeTargets(t *testing.T) {
 	root := t.TempDir()
 	outside := t.TempDir()