fix(alerts): one silent agent is one alert and one finding (#5721)

VDNT-Docker03's silent agent read as three overview findings (agent offline,
agent stale, the disk rule's no-data) and every drop emailed the disk rule's
silence and return on top of agent offline.

- The host agent registers its heartbeat (seconds_since_seen) and the
  sources it carries (host_agent, docker). While a host's agent is past its
  offline window, its other rules' no_data is recorded "while its agent is
  offline" and not emailed, nor is their quiet return; a return past a
  threshold still fires.
- The overview folds those rules under the host's agent-offline finding
  ("also silent with it: ..."), and a stale agent whose agent-offline alert
  is active is that alert's finding, not a second one.

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

plugins/host_agent/__init__.py

@@ -19,6 +19,12 @@ def setup(app: "Quart") -> None:
     learn_normal("host_agent", "cpu_pct", floor_setting(app, "host_cpu_pct"), parts=False)
     learn_normal("host_agent", "mem_used_pct", floor_setting(app, "host_mem_pct"), parts=False)
     learn_normal("host_agent", "load_pct", floor_setting(app, "host_load_pct"), parts=False)
+    # "Agent offline" speaks for everything the agent carries: while it is
+    # firing, the host's other rules going silent isn't emailed (#5721).
+    from steward.core.alerts import agent_fed, agent_heartbeat
+    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])
 
 
 def get_scheduled_tasks() -> list: