Provide the ability to rollback an import

Provide a method for importers to rollback its changes, possibly with an interactive dialog upon failure to import all/any of the content items.

Should also have a command to be handy for testing.

This can use the canonical id field to get the list of imported content, or importers can add a meta key during import, and get rid of it ( via simple sql query ) after import has finished, to indicate items related to the current/just-finished import job. Importers should clear this meta from all posts before beginning a new import process.

Start by inspecting the importer lifecycle and how canonical IDs or post meta identify content from an import. Clarify the rollback behavior for partial and failed imports, including the proposed interactive dialog and testing command. Done means an import can reliably undo its changes and the rollback path is testable.

Written by the indexing model from the issue text.