fix: missing fmt import in auth.go and writeFile helper in git.go

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

backend/internal/git/git.go

@@ -3,6 +3,7 @@ package git
 import (
 	"bytes"
 	"fmt"
+	"os"
 	"os/exec"
 	"path/filepath"
 	"strings"
@@ -92,6 +93,13 @@ type LogEntry struct {
 	Subject string
 }
 
+func writeFile(path, content string) error {
+	if err := os.MkdirAll(filepath.Dir(path), 0755); err != nil {
+		return err
+	}
+	return os.WriteFile(path, []byte(content), 0644)
+}
+
 func (r *Repo) run(args ...string) (string, error) {
 	cmd := exec.Command("git", append([]string{"-C", r.root}, args...)...)
 	var stdout, stderr bytes.Buffer