mara's review on #2436: no submit-await-submit composition, even
server-side. Adds Template::GracefulRestart (Signal -> Drain ->
StopForUpdate -> Reconcile, wanted=Up) mirroring how Restart already
does StopForUpdate -> Reconcile, plus submit::graceful_restart and
templates::graceful_restart. handle_restart_scoped now submits exactly
one DAG per agent up front for both the graceful and non-graceful
case -- no await_dags in the loop anymore.
hivectl restart --agent NAME previously composed stop() then start() as
two separate client-side daemon calls glued by CLI-process control flow
— not one DAG, and a dropped hivectl connection mid-restart (ssh drop,
Ctrl-C) left the agent stopped with no automatic follow-up. mara flagged
this as the first target for the 'dagify hivectl commands' issue.
New HostRequest::RestartScoped{scope, graceful} handles it server-side:
each targeted agent now rides exactly one atomic Restart-template DAG
(same one hivectl agents restart / restart-all already use) in the
common non-graceful case. --graceful has no single-DAG template yet, so
it submits the graceful-stop DAGs, awaits them server-side, then submits
the start DAGs — still one daemon call end to end, just not yet a single
DAG (noted as a follow-up). Infra containers restart synchronously as
before (no lease/DAG concept for them).
CLI-side restart() now just makes the one call + waits, same output
shape as before via render_lifecycle.
