When the gateway shuts down gracefully (hermes update, gateway restart,
/restart), it now writes a .clean_shutdown marker file. On the next
startup, if this marker exists, suspend_recently_active() is skipped
and the marker is cleaned up.
Previously, suspend_recently_active() fired on EVERY startup —
including planned restarts from hermes update or hermes gateway restart.
This caused users to lose their conversation history unexpectedly: the
session would be marked as suspended, and the next message would
trigger an auto-reset with a notification the user never asked for.
The original purpose of suspend_recently_active() is crash recovery —
preventing stuck sessions that were mid-processing when the gateway
died unexpectedly. Graceful shutdowns already drain active agents via
_drain_active_agents(), so there is no stuck-session risk. After a
crash (no marker written), suspension still fires as before.
Fixes the scenario where a user asks the agent to run hermes update,
the gateway restarts, and the user's next message gets an unwanted
'Session automatically reset' notification with their history cleared.
