feat(alerts): pick the host, then its part, and see what the metric reads (#5475)

#5475 "Rule form: pick the host, then its part; metrics that part reports;
a link to groups; a preview of the metric", from the operator's review of
the New rule page: hosts and their parts shared one long picker.

- Host agent rules pick a Host, then a Part: "The whole host", then its
  mounts, CPU cores, disks, interfaces and sensors under a heading each,
  cores in number order. A part that reports nothing a rule can watch
  (an interface's traffic) isn't offered.
- The Metric list holds what the picked resource reports, in catalog order.
- Under it, a preview: the reading now, and the last 24 h low to high with
  an hourly sparkline drawn from zero (UI ruling 8), read from one series.
- The group picker links to Ansible > Inventory to make a group, in a new
  tab, and re-reads the groups when the tab comes back into view.
- Picking a host or part swaps the pickers alone (/alerts/api/rule_target);
  picking a metric swaps the preview (/alerts/api/metric_preview).
- A rule's own resource and metric stay on offer after they go quiet.

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

steward/templates/alerts/rules_form.html

@@ -56,9 +56,7 @@
 
       {# ── Dynamic resource + metric ────────────────────────────────────────── #}
       <div id="rule-fields">
-        {% with current_resource = rule.resource_name if rule else "", current_metric = rule.metric_name if rule else "" %}
         {% include "alerts/_rule_fields.html" %}
-        {% endwith %}
       </div>
       {# ── Metric reference ────────────────────────────────────────────────── #}
       <details style="margin-bottom:1rem;">