[fix] VFS singleton reset + remove vestigial check_username tool

- VFS now creates fresh instance when root_path/fs_data is provided
  (prevents cross-scenario contamination in multi-scenario runs)
- Removed check_username from tools (unrelated to any scenario)

1 files changed, 3 insertions, 1 deletions

diff --git a/src/vfs.py b/src/vfs.py
index 2bbfdc3..0b3785b 100644
--- a/src/vfs.py
+++ b/src/vfs.py
@@ -114,7 +114,9 @@ class VFS:
 
     @classmethod
     def get_instance(cls, root_path=None, fs_data=None):
-        if cls._instance is None:
+        if root_path is not None or fs_data is not None:
             cls._instance = VirtualFileSystem(root_path, fs_data)
+        if cls._instance is None:
+            cls._instance = VirtualFileSystem()
         return cls._instance