update_product

Update the product header (`$upg.product`): stage, title, description, health_status, url, and the workspace member_kind. The supported way to advance a product's lifecycle stage or re-kind a graph; it writes the value get_graph_digest reads, without hand-editing the integrity-hashed .upg file. A title rename or a re-kind also reconciles the workspace.json cache and the portfolio.upg registry, so list_local_products, get_workspace_info, portfolio_census, counts.products, and the watched anti-pattern scoping all show the current value. Set rename_file (or pass an explicit slug) to also rename the .upg file to match the title: it moves the file, repoints the open handle so the rest of the session writes to the new path, and updates the workspace.json file path and the portfolio.upg file_path. The rename is opt-in; a plain title change leaves the filename alone.