feat(docker): Docker's own status reaches Steward — an empty answer clears a host, a lost one greys it, a demoted manager's swarm rows go (#5152)

The agent (1.9.0) said nothing when Docker could not be asked and nothing
when Docker had no containers, so Steward could tell neither apart from the
other, and kept a host's last containers on screen as running either way.

- Agent: collect_docker returns (status, detail, containers). "ok" sends
  docker: [] when there are none; "unavailable" (no socket, nothing
  listening) and "error" (permission, timeout, bad answer) say why.
  collect_swarm says whether /info answered, and a node that is not a
  manager sends swarm_manager: false. A failed /info says nothing.
- Ingest: an empty docker list is a snapshot; the newest sample's status
  goes to the Docker plugin with the rest.
- docker_host_status (docker_011): per host, status, detail, since and
  last report. A host that never had Docker gets no row.
- An "ok" empty snapshot reaps the host's containers; an error keeps them.
  swarm_manager: false drops the host's swarm services and nodes.
- Docker page: a "Docker not answering" card names each such host, why,
  and since when; its containers read as stale. Duplicate header comment
  in rows.html removed.

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

plugins/docker/templates/docker/rows.html

@@ -1,5 +1,3 @@
-{# docker/rows.html — HTMX fragment for the Docker main page, grouped by host #}
-
 {# docker/rows.html — HTMX fragment for the Docker page: what needs attention
    first (service.needs_attention, the ranking the MCP tools use), the counts,
    then swarm services and each host's containers. #}
@@ -28,6 +26,22 @@
 </section>
 {% endif %}
 
+{# ── Hosts whose Docker is not answering (agent ≥ 1.9.0) ──────────────────── #}
+{% if docker_down %}
+<section class="card problem-card" aria-labelledby="docker-unreachable">
+  <h2 id="docker-unreachable" class="section-title" style="margin-bottom:0.6rem;">Docker not answering</h2>
+  <ul class="problem-list">
+    {% for d in docker_down %}
+    <li>
+      {{ status_badge('crit', 'Error') if d.status == 'error' else status_badge('stale', 'No Docker') }}
+      <a href="/hosts/{{ d.host_id }}">{{ d.host_name }}</a>
+      <span class="problem-detail">{{ d.detail or "no reason given" }} · since {{ d.since|ago }} · its containers below are what Docker last reported</span>
+    </li>
+    {% endfor %}
+  </ul>
+</section>
+{% endif %}
+
 {# ── Counts ───────────────────────────────────────────────────────────────── #}
 <div class="stat-strip">
   {{ stat_tile("Needs attention", attention|length, state="crit" if attention else "inactive") }}