How can I add a custom column to the “Manage Categories” table?
The filter is manage_{$screen->id}_columns, and $screen->id is edit-category, giving you manage_edit-category_columns. I found this by placing a var_dump() in get_column_headers(), which is called by print_column_headers(), which is called in wp-admin/edit-tags.php, the page where you edit the category items.