Check housekeeping has cleared up

The `housekeeping` task runs after `standardise_model_data` in order to remove the extracted PP files from the installed workflow. We should have a check that there are no remaining PP files (or potentially any unwanted netCDF files) left after the housekeeping task has run. This could be done as a standalone task or as a [post-script](https://cylc.github.io/cylc-doc/stable/html/reference/config/workflow.html#flow.cylc[runtime][%3Cnamespace%3E]post-script) to the `housekeeping` task.

Locate the workflow configuration for the housekeeping task and the preceding standardise_model_data task, then review how the post-script option is used. Add a check that confirms extracted PP files, or other unwanted netCDF files if that scope is chosen, are absent after housekeeping completes; done means the workflow reports failure when files remain.

Written by the indexing model from the issue text.