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/docker/__init__.py

@@ -48,6 +48,9 @@ def setup(app: "Quart") -> None:
     from steward.core.norms import floor_setting, learn_normal
     learn_normal("docker", "cpu_pct", floor_setting(app, "container_cpu_pct"))
     learn_normal("docker", "mem_pct", floor_setting(app, "container_mem_pct"))
+    # A host's agent carries its containers' figures: they go silent with it (#5721).
+    from steward.core.alerts import agent_fed
+    agent_fed("docker", lambda resource: resource.split("/", 1)[0])
 
 
 def get_scheduled_tasks() -> list: