refactor(auth): replace sanitizeAntigravityFileName with antigravity.CredentialFileName

internal/auth/antigravity/filename.go

@@ -0,0 +1,16 @@
+package antigravity
+
+import (
+	"fmt"
+	"strings"
+)
+
+// CredentialFileName returns the filename used to persist Antigravity credentials.
+// It uses the email as a suffix to disambiguate accounts.
+func CredentialFileName(email string) string {
+	email = strings.TrimSpace(email)
+	if email == "" {
+		return "antigravity.json"
+	}
+	return fmt.Sprintf("antigravity-%s.json", email)
+}