num_run was declared in the host model but never set anywhere — the
controller's worker push carries no running-job count, so hosts always
showed 0 regardless of running jobs. Compute it live in the hosts list
handler by counting RUN jobs per exec host from store.Jobs — the same
source the jobs list uses, so the two stay consistent.
Verified: t1 with 5 RUN jobs shows num_run 5, matching the jobs list.
Co-Authored-By: Claude <noreply@anthropic.com>
