chore: simplify and fix processed data gitignore rule

Replace five narrow patterns (*.pt, *.pkl, train/, val/, holdout/) with
a single data/processed/ rule that also covers data/processed/user/.
All processed tensors are reproducible from committed .chord files.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -33,11 +33,7 @@ checkpoints/*.pth
 checkpoints/*.ckpt
 
 # Processed data (reproducible from source)
-data/processed/*.pt
-data/processed/*.pkl
-data/processed/train/
-data/processed/val/
-data/processed/holdout/
+data/processed/
 
 # External corpora (download separately; too large for git)
 data/raw_external/