Add configurable maintenance mode file support

Implement maintenance_mode_file configuration option in NotificationConfig
to allow customizable file paths for suppressing email notifications.
Updates maintenance mode check to use configured path instead of hardcoded
/tmp/cm-maintenance file.

agent/src/notifications/mod.rs

@@ -59,6 +59,6 @@ impl NotificationManager {
     }
 
     fn is_maintenance_mode(&self) -> bool {
-        std::fs::metadata("/tmp/cm-maintenance").is_ok()
+        std::fs::metadata(&self.config.maintenance_mode_file).is_ok()
     }
 }