fix(#878): add robust crontab binary check to requirements

tools/cronjob_tools.py

@@ -369,9 +369,13 @@ def check_cronjob_requirements() -> bool:
     """
     Check if cronjob tools can be used.
 
+    Requires 'crontab' executable to be present in the system PATH.
     Available in interactive CLI mode and gateway/messaging platforms.
-    Cronjobs are server-side scheduled tasks so they work from any interface.
     """
+    # Fix for issue #878: ensure crontab binary is actually available
+    if not shutil.which("crontab"):
+        return False
+
     return bool(
         os.getenv("HERMES_INTERACTIVE")
         or os.getenv("HERMES_GATEWAY_SESSION")