feat(dashboard): per-metric sparklines in the Host Agent — Resources widget

Operator ask: show the graphs next to their fields in the fleet widget, like the
host page's AGENT panel. Each row now renders cpu/mem/disk/load as a value with a
trend sparkline beneath it (1h window), instead of bare numbers.

- _fleet_rows fetches a per-host recent series (cpu/mem/load host-level, disk from
  the root mount) in one 1h query and attaches a sparkline per metric to each row.
- widget_table.html lays out a metric cell (label + threshold-coloured value +
  sparkline) per field, mirroring panel.html. Threshold colour is computed in the
  loop and passed into the cell macro (keeps Jinja macro scope clean).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

steward/core/widgets.py

@@ -270,7 +270,7 @@ WIDGET_REGISTRY: dict[str, dict] = {
     "host_resources": {
         "key": "host_resources",
         "label": "Host Agent — Resources",
-        "description": "Fleet view: CPU, memory, disk, and load per monitored host",
+        "description": "Fleet view: CPU, memory, disk, and load per host — each with a trend sparkline",
         "hx_url": "/plugins/host_agent/widget",
         "detail_url": "/hosts/",
         "plugin": "host_agent",