feat(docker): admin-gated per-host prune buttons on the image/disk page

M78 step 2. The /disk page surfaced reclaimable space + stopped counts but
said "prune deferred to a later milestone" — this wires the cleanup.

Adds POST /plugins/docker/disk/<host_id>/prune (admin only) that resolves
the host's linked AnsibleTarget → steward:target:<id> scope and fires the
bundled maintenance/docker_prune.yml through the ansible.run_playbook
capability (audited runner; the collection agent stays read-only). Mirrors
the host_agent update route. `target` ∈ {containers, images, system}:
- containers → docker container prune -f
- images     → docker image prune -af (prune_all_images)
- system     → docker system prune -f (conservative)

The disk() route now resolves each host group's linked target + capability
availability; disk.html renders three confirm-gated buttons per host, shown
only to admins when Ansible is available and the host has a linked target,
otherwise a clear "link a target" / "runner unavailable" hint (rules 24/26/27).

Tests: the prune_target→extra_vars mapping (the -af-only-for-images rule) is
extracted to a pure helper and unit-tested; disk_prune added to the module
smoke; disk.html covered by the existing template-parse test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CAGR73DUowdVFVvYzLXC5C

plugins/docker/templates/docker/disk.html

@@ -7,8 +7,8 @@
 
 <h1 class="page-title" style="margin-bottom:0.4rem;">Image &amp; disk usage</h1>
 <p style="font-size:0.82rem;color:var(--text-muted);margin-bottom:1.5rem;">
-  Reclaimable = space held by images no container references. Cleanup actions
-  (prune) arrive in a later release — these are read-only figures for now.
+  Reclaimable = space held by images no container references. Admins can prune
+  per host below; each action runs an audited Ansible playbook on that host.
 </p>
 
 {% if host_groups %}
@@ -47,6 +47,43 @@
     </div>
   </div>
 
+  {# ── Cleanup actions (admin only; audited Ansible prune run) ───────────── #}
+  {% if session.user_role == 'admin' %}
+  <div style="margin-bottom:1rem;">
+    {% if ansible_available and g.has_target %}
+    <div style="display:flex;flex-wrap:wrap;gap:0.5rem;align-items:center;">
+      <form method="post" action="/plugins/docker/disk/{{ g.host_id }}/prune" style="margin:0;"
+            data-msg="Remove all STOPPED containers on {{ g.host_name|e }}? This cannot be undone."
+            onsubmit="return confirm(this.dataset.msg);">
+        <input type="hidden" name="target" value="containers">
+        <button type="submit" class="btn btn-sm">Prune stopped containers</button>
+      </form>
+      <form method="post" action="/plugins/docker/disk/{{ g.host_id }}/prune" style="margin:0;"
+            data-msg="Remove ALL unused images on {{ g.host_name|e }} (docker image prune -a)? Any image not used by a container is deleted."
+            onsubmit="return confirm(this.dataset.msg);">
+        <input type="hidden" name="target" value="images">
+        <button type="submit" class="btn btn-sm">Prune unused images</button>
+      </form>
+      <form method="post" action="/plugins/docker/disk/{{ g.host_id }}/prune" style="margin:0;"
+            data-msg="Run a full system prune on {{ g.host_name|e }}? Removes stopped containers, unused networks, dangling images and build cache."
+            onsubmit="return confirm(this.dataset.msg);">
+        <input type="hidden" name="target" value="system">
+        <button type="submit" class="btn btn-sm btn-danger">System prune…</button>
+      </form>
+    </div>
+    <div style="font-size:0.72rem;color:var(--text-muted);margin-top:0.4rem;">
+      Reclaimed space appears on the next agent sample.
+    </div>
+    {% elif not ansible_available %}
+    <div style="font-size:0.74rem;color:var(--text-muted);">Cleanup needs the Ansible runner (currently unavailable).</div>
+    {% else %}
+    <div style="font-size:0.74rem;color:var(--text-muted);">
+      Link an <a href="/hosts/{{ g.host_id }}">Ansible target</a> to this host to enable prune actions.
+    </div>
+    {% endif %}
+  </div>
+  {% endif %}
+
   {# ── Per-image table ──────────────────────────────────────────────────── #}
   <div class="card-flush">
     <table class="table">