fix: truncate training log file before each run

Updated init_logging in training_lib.sh to truncate the log file
before each new training run using ': > '.

This prevents historical errors from accumulating in the log file,
which was causing the validation to count old failures and report
inaccurate success rates (e.g., 87% when current run was ~99%).

Now each run starts with a fresh log file containing only the current
run's output, making validation results accurate.

scripts/training/training_lib.sh

@@ -55,6 +55,9 @@ init_logging() {
     mkdir -p "$LOG_DIR"
     export CURRENT_LOG="$log_file"
     
+    # Truncate log file to only contain current run
+    : > "$log_file"
+    
     {
         echo "========================================"
         echo "AITBC Training - $stage_name"