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>

docs/core/alerting.md

@@ -75,6 +75,15 @@ Alert rules are created in the UI at `/alerts/`. Each rule requires:
 - **Threshold** — numeric value
 - **Consecutive failures required** — how many consecutive breaches before FIRING (default 1)
 
+The form offers only what has reported, read from `plugin_metric_latest`.
+For the host agent, a **Host** is picked first, then a **Part** of it: "The
+whole host", then its mounts, CPU cores, disks and so on, under a heading for
+each kind. A part that reports nothing a rule can watch, such as an interface's
+traffic, isn't listed. The **Metric** list holds only what the picked resource
+reports. Under it, a preview shows the metric's current reading and its last
+24 hours (lowest to highest, with an hourly sparkline), so the threshold can be
+set against real numbers.
+
 ### Rules that cover hosts
 
 A rule on `host_agent` can cover **every host**, or **every host in an