feat(host_agent): a reboot reads as a reboot, and lost contact as its span (#5756)

#5756 "The agent says when its host rebooted and when it lost contact, so
downtime is recorded as what it was". Agent 1.14.0 posts events with the
next post that gets through: started (with the host's boot time), stopping
(on SIGTERM, with whether systemd says the host is going down) and
lost_contact (from its first unanswered post to the one carrying it).

Steward keeps them as the host's downtime (host_downtime): reboot,
agent_stop, unclean (a boot after the agent was last heard that nobody
announced) or lost_contact. Retried posts apply once.

An announced stop explains the silence for reboot_grace_seconds (Settings
-> Plugins -> host_agent, 10 minutes): seconds_since_seen reads 0 until
then, and the host's other rules go quiet with it (agents_away beside
#5721's registry). Past the grace, agent offline fires.

The host's Agent card lists the last 7 days of downtime. Rows are pruned
at data.retention_days.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

plugins/host_agent/__init__.py

@@ -25,6 +25,15 @@ def setup(app: "Quart") -> None:
     from steward.hosts.service import agent_stale_after_seconds
     agent_heartbeat("host_agent", "seconds_since_seen", lambda: agent_stale_after_seconds(app.config))
     agent_fed("host_agent", lambda resource: resource.split(":", 1)[0])
+    # A reboot the agent announced is silence with a reason, for a while (#5756).
+    from datetime import datetime, timezone
+    from steward.core.alerts import agents_away
+    from .downtime import hosts_away, restart_grace_seconds
+    agents_away(lambda session: hosts_away(session, datetime.now(timezone.utc),
+                                           restart_grace_seconds(app.config)))
+    from steward.core.cleanup import prune_by_age
+    from .models import HostDowntime
+    prune_by_age(HostDowntime.started_at)
 
 
 def get_scheduled_tasks() -> list: