Return this from a Permissions can_use_tool callback to suspend before
execution. The Agent must have an approval_dir. Resumable tools must be
registered with convert = FALSE; their functions accept raw JSON arguments.
Tool outputs should be strings, explicit JSON, or ellmer Content values.
Raw JSON shaped like a content record (version, class, props) is rejected
because ellmer replay would reinterpret it as an S7 constructor.
No approval is granted by constructing this value.