run() previously buffered the child's output via .output() and
only logged at exit — a multi-minute 'nixos-container update'
(typical on a fresh hyperhive bump) showed nothing in journald
until the very end. operator watching 'journalctl -u hive-c0re
-f' couldn't tell 'slow nix build' from 'wedged daemon'.
new shape: spawn with piped stdio, pump each line into tracing
as it arrives (stdout → INFO, stderr → WARN), keep a tail of
the last 32 stderr lines for the bail message so the eventual
'failed (status 2)' still carries the actual nix eval error.
target field 'nixos-container', argv-equivalent attached via
the 'cmdline' field so filtering by subcommand works.
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