Fix tests modifying test fixture files by using temporary directories

Previously, running tests would modify test/config/Users/guest.yaml and create
test/config/Users/test-user.yaml due to the automatic migration logic in
YAMLAccountManager. This caused git diff to show changes after running tests.

Solution:
- Modified TestNewYAMLAccountManager to use t.TempDir() for test isolation
- Added copyTestFiles helper to copy test fixtures to temporary directory
- Tests now run against copies, leaving original fixtures untouched

This ensures tests are properly isolated and don't have side effects on the
repository's test fixture files.

0 files changed, 0 insertions, 0 deletions