refactor: Use threshold variable consistently

The `config.Diff.Threshold` variable was being used directly in log messages and conditional statements. This commit introduces a new variable `threshold` to store the value of `config.Diff.Threshold` and uses it consistently throughout the `rootCmd` function. This improves code readability and maintainability by reducing redundant access to the configuration value.
---
{"auto-commit-msg":{"language":"rust","version":"0.4.0-dev","model":"gemini-2.5-flash-lite","response_time":1.08,"execution_time":1.1}}

Author: sestrella

cmd/root.go

@@ -93,14 +93,14 @@ var rootCmd = &cobra.Command{
 		}
 
 		totalChanges := stats.Insertions + stats.Deletions
-		totalChangesThreshold := config.Diff.Threshold
+		threshold := config.Diff.Threshold
 		var model string
-		if totalChanges < totalChangesThreshold {
+		if totalChanges < threshold {
 			model = config.Diff.ShortModel
-			log.Printf("git diff total changes %d under %d threshold, using model for short diffs: %s\n", totalChanges, totalChangesThreshold, model)
+			log.Printf("git diff total changes %d under %d threshold, using model for short diffs: %s\n", totalChanges, threshold, model)
 		} else {
 			model = config.Diff.LongModel
-			log.Printf("git diff total changes %d over %d threshold, using model for long diffs: %s\n", totalChanges, totalChangesThreshold, model)
+			log.Printf("git diff total changes %d over %d threshold, using model for long diffs: %s\n", totalChanges, threshold, model)
 		}
 		if config.Provider.ApiKey == "" {
 			return errors.New("api_key environment variable name cannot be empty")