A restart now interrupts in-flight agents immediately rather than holding
the gateway open for a grace window. The previous 180s default coupled two
independently-set timers: the gateway's own drain timer and systemd's
TimeoutStopSec. On a stale unit where TimeoutStopSec < drain, systemd
SIGKILLed the gateway mid-cleanup, leaving a stale lock that made the next
startup exit immediately ('already running') — an infinite crash loop under
Restart=on-failure (#31981).
Setting drain to 0 makes the mismatch structurally impossible: with drain 0
the generated unit gets TimeoutStopSec=90 against a near-instant drain, so
systemd never kills mid-cleanup. Contract: restart the gateway, in-flight
work stops. A grace window large enough to 'save' a long agent turn would
have to outlast an unbounded task, which is impossible.
Also fixes the stale-unit warning's suggested command
(hermes gateway service install --replace -> hermes gateway install --force);
the former subcommand does not exist.
Closes#31981
c9df4bc094