HostRequest::Rebuild was the only rebuild path that bypassed
notify_manager. dashboard / auto_update / actions::approve
already emit Rebuilt events on both success + failure, but a
'hive-c0re rebuild <name>' from the host CLI (and the recent
matrix-flake build failure that surfaced in journald) left the
manager in the dark.
mirror auto_update::rebuild_agent's pattern: on success →
Rebuilt{ok:true}, on failure → Rebuilt{ok:false, note=
format!('{e:#}')}. note carries the stderr tail lifecycle::run
collected (the actual nix error: missing prompt file, dep
build failure, etc.), so the manager has enough context to
adjust the agent's agent.nix without ssh-ing to the host.
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