feat(docker): per-host collection via the host agent; drop central scrape

Docker collection moves off the central single-socket scrape onto the host
agent, giving Docker a real per-host dimension. The Steward host now reports
its own containers like any other host, and same-named containers on different
hosts no longer collide.

- agent: stdlib UDS Docker client (AF_UNIX HTTP/1.1, Connection: close,
  chunked-aware), collect_docker() ports the cpu%/mem math; sample["docker"]
  added best-effort (silent-skip on absent/unreadable socket). AGENT_VERSION
  1.2.0 → 1.3.0; optional docker_socket config key.
- ingest: host_agent ingest hands per-host container snapshots to the docker
  plugin via a new "docker.persist_host_samples" capability (no hard import,
  no-op when docker disabled), inside a SAVEPOINT so a docker failure never
  sinks the host metrics. Resource names are host-scoped ("<host>/<name>").
- schema: docker_containers re-keyed (host_id, name); docker_metrics gains
  host_id; docker_002 migration DROP+recreates (dev-only, rule 122).
- ui: Docker page + widgets grouped by host with host links; new per-host
  Docker panel embedded on the Hosts hub (gated on docker enabled via a new
  enabled_plugins template context). Replaces the SQLite-only strftime
  bucketing with DB-agnostic Python bucketing.
- provisioning: install/provision playbooks add steward-agent to the docker
  group (best-effort) so the agent can read the socket.
- removed central scrape: docker scheduler.py + scraper.py deleted; plugin.yaml
  socket_path/scrape_interval_seconds/include_stopped dropped (plugin 2.0.0).
- tests: agent docker collector units (math, chunked decode, silent-skip,
  sample shape, config) + integration (host-scoped schema + persistence).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016Jg27rgypiW2efULXJDtMC

plugins/docker/templates/docker/rows.html

@@ -1,4 +1,4 @@
-{# docker/rows.html — HTMX fragment for the Docker main page #}
+{# docker/rows.html — HTMX fragment for the Docker main page, grouped by host #}
 
 {# ── Summary strip ─────────────────────────────────────────────────────────── #}
 <div style="display:grid;grid-template-columns:repeat(auto-fill,minmax(140px,1fr));gap:0.75rem;margin-bottom:1.5rem;">
@@ -12,13 +12,28 @@
   </div>
   <div class="card" style="margin-bottom:0;">
     <div class="section-title" style="margin-bottom:0.35rem;">Total</div>
-    <span class="stat-val">{{ container_data | length }}</span>
+    <span class="stat-val">{{ total }}</span>
+  </div>
+  <div class="card" style="margin-bottom:0;">
+    <div class="section-title" style="margin-bottom:0.35rem;">Hosts</div>
+    <span class="stat-val">{{ host_groups | length }}</span>
   </div>
 </div>
 
-{# ── Container table ─────────────────────────────────────────────────────── #}
-{% if container_data %}
-<div class="card-flush">
+{# ── Container tables, one per host ───────────────────────────────────────── #}
+{% if host_groups %}
+{% for g in host_groups %}
+<div class="card-flush" style="margin-bottom:1.5rem;">
+  <div style="display:flex;align-items:baseline;gap:0.6rem;padding:0.5rem 0.75rem;border-bottom:1px solid var(--border);">
+    {% if g.host %}
+    <a href="/hosts/{{ g.host.id }}" style="font-weight:600;font-size:0.95rem;color:inherit;text-decoration:none;">{{ g.host.name }}</a>
+    {% else %}
+    <span style="font-weight:600;font-size:0.95rem;">{{ g.host_name }}</span>
+    {% endif %}
+    <span style="font-size:0.78rem;color:var(--text-muted);">
+      {{ g.running }} running{% if g.stopped %} · {{ g.stopped }} stopped{% endif %}
+    </span>
+  </div>
   <table class="table">
     <thead>
       <tr>
@@ -32,7 +47,7 @@
       </tr>
     </thead>
     <tbody>
-      {% for item in container_data %}
+      {% for item in g.containers %}
       {% set c = item.container %}
       <tr>
         <td>
@@ -77,10 +92,13 @@
     </tbody>
   </table>
 </div>
+{% endfor %}
 {% else %}
 <div class="card" style="text-align:center;padding:3rem;">
   <div style="color:var(--text-muted);font-size:0.9rem;">
-    No containers found. Make sure the Docker socket is accessible and the plugin is configured correctly.
+    No containers reported yet. Containers are collected by the Steward host agent —
+    deploy the agent to a host running Docker (Hosts → the host → agent panel) and
+    its containers will appear here under that host.
   </div>
 </div>
 {% endif %}