Add a generic suppress_notification flag to the drain-request marker. When a
drain that ends in process exit (e.g. a NAS auto-update image migration on the
always-on Hermes Cloud fleet) is flagged, the gateway skips ONLY the
home-channel 'gateway shutting down' broadcast — the operator-flavoured ping
that would otherwise fire on every routine auto-update, dozens of times a day.
The per-active-session interrupt ping is ALWAYS kept: on a drained shutdown
it's empty by construction, and in the force-interrupt (deadline-exceeded) case
it carries the user-valuable 'your task was cut off, message me to resume' hint.
The gateway stays agnostic about WHY a drain is quiet (generic boolean, not a
kind enum); the policy of which drain causes set the flag lives in the caller
(NAS). Default-false so legacy/operator drains behave exactly as before. The
reader reuses the NS-570 epoch-staleness check so an orphaned marker on the
durable volume can never silence a fresh gateway's legitimate broadcast.
- drain_control.py: write_drain_request gains suppress_notification; new
drain_notification_suppressed() reader (current-epoch + truthy flag).
- web_server.py: /api/gateway/drain reads + echoes the flag.
- run.py: _notify_active_sessions_of_shutdown skips the home-channel loop only.
Tests prove: flag round-trips; home-channel suppressed when set, kept when
unset; active-session ping always fires; stale/legacy/corrupt markers never
suppress.
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