fix: check file existence during wizard instead of only after

Move the os.Stat existence check into validateYAMLExtension so users
get immediate feedback at stepSaveFile time, avoiding lost work if the
target file already exists.

Author: dangrondahl

internal/policywizard/forms.go

@@ -2,6 +2,7 @@ package policywizard
 
 import (
 	"fmt"
+	"os"
 	"path/filepath"
 	"regexp"
 	"slices"
@@ -264,6 +265,9 @@ func validateYAMLExtension(s string) error {
 	if ext != ".yaml" && ext != ".yml" {
 		return fmt.Errorf("file must have a .yaml or .yml extension")
 	}
+	if _, err := os.Stat(s); err == nil {
+		return fmt.Errorf("file %q already exists; choose a different name", s)
+	}
 	return nil
 }