fix: allow golden regression data in git while ignoring other pkl files

- Update .gitignore to explicitly allow golden_data/*.pkl files
- Add exception pattern after *.pkl rule
- Commit existing golden data files (clusterless_decoder, sorted_spikes_decoder)
- Golden data should be version controlled per regression testing design

Author: edeno

.gitignore

@@ -155,6 +155,8 @@ _version.py
 .DS_Store
 *.npz
 *.pkl
+# Exception: commit golden regression data for version control
+!src/non_local_detector/tests/golden_data/*.pkl
 *.png
 
 # Git worktrees