Summary view of a run’s checkpoint — the query DTO shared by the CLI.
It lives in the query layer (not a presentation layer) so that
the binary commands depend downward on service, not the reverse.
Fetch a run’s checkpoint, guarding against unknown run dirs: returns
Ok(None) rather than letting get_run_store create the run directory for
an id that was never started. The single existence-checked accessor shared
by list_runs / get_status and the binary status command.