hermes update no longer dies when the controlling terminal closes
(SSH drop, shell close) during pip install. SIGHUP is set to SIG_IGN
for the duration of the update, and stdout/stderr are wrapped so writes
to a closed pipe are absorbed instead of cascading into process exit.
All update output is mirrored to ~/.hermes/logs/update.log so users can
see what happened after reconnecting.
SIGINT (Ctrl-C) and SIGTERM (systemd) are intentionally still honored —
those are deliberate cancellations, not accidents. In gateway mode the
helper is a no-op since the update is already detached.
POSIX preserves SIG_IGN across exec(), so pip and git subprocesses
inherit hangup protection automatically — no changes to subprocess
spawning needed.
