feat: harden agent runtime for long-running tasks

nanobot/agent/tools/shell.py

@@ -52,6 +52,10 @@ class ExecTool(Tool):
     def description(self) -> str:
         return "Execute a shell command and return its output. Use with caution."
 
+    @property
+    def exclusive(self) -> bool:
+        return True
+
     @property
     def parameters(self) -> dict[str, Any]:
         return {