Process Files Open

A file's content, for the workspace file viewer — the read counterpart to list/list_abs above. Registered ahead of the mutate catch-all below (FastAPI matches by registration order, and {action} would otherwise swallow this literal path first) since open is core's one file-content route that is itself a POST (it takes the path in a JSON body, not a query string) rather than because it belongs in the mutate allow-list.