Use vm/ instead of the hidden .vm/ as the default state directory

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

.gitignore

@@ -1,5 +1,5 @@
-# QEMU runtime state and writable overlays
-.vm
+# QEMU runtime state and writable overlays (vm/ in the repository root)
+/vm/
 
 # VM disk and generated installation/seed images
 *.qcow2