A hive-wide restart was N separate single-agent DAGs (one submit::restart
per agent). Now that agent is per-node (#2445), make it ONE DAG with a
per-agent restart subgraph each.
- templates::restart takes an agent list: each agent gets an independent
subgraph (a head SetWanted(Up) root, then its restart chain), so the N
subgraphs run concurrently on their own leases. One agent = the ordinary
single-agent restart; unifies the old restart + graceful_restart fns.
- submit::restart / graceful_restart stay as single-agent wrappers over
the new submit::restart_many(agents, graceful).
- server.rs handle_restart_all + handle_restart_scoped submit one
restart_many call instead of looping per agent. Infra containers
unchanged (no lease/DAG, synchronous).
Scope: restart + restart-all only. Broad stop+start is the same pattern
(stop/start templates take agent lists) — a follow-up increment.
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