bare set_transient/clear_transient pairs leak the in-memory transient
on task cancellation, panics, or any early return between the two
calls — dashboard then shows the agent stuck in 'rebuilding…'
forever (coder hit this today). add Coordinator::transient_guard
returning a TransientGuard whose Drop clears, and convert every
caller (dashboard lifecycle_action, auto_update::rebuild_agent,
manager_server Update, actions::destroy, actions Spawn task,
migrate phase 4). destroy() now takes &Arc<Coordinator> so it can
hold a guard. existing stuck transients clear on next hive-c0re
restart since transient state is in-memory only.
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