chore(env): remove outdated GRAIL_CHECKPOINT_RETENTION_LIMIT from example configuration

Author: erfanMhi

.env.example

@@ -249,8 +249,6 @@ GRAIL_GRPO_RANKING_VARIANCE_WEIGHT=0.3
 # ============================================================================
 # GRAIL CHECKPOINT CONFIGURATION
 # ============================================================================
-# Number of checkpoints to keep on R2 (default: 10)
-GRAIL_CHECKPOINT_RETENTION_LIMIT=3
 
 # Interval for milestone checkpoints (every N windows, default: 100)
 GRAIL_CHECKPOINT_MILESTONE_INTERVAL=100