feat(host_agent): at-a-glance disk = root (/) not "worst"; metric tooltips

The "Disk (worst)" number was opaque at a glance. Show the root filesystem (/)
usage instead — what people actually care about — on the host panel and the
dashboard widget. "Worst" is kept only for the widget's health dot (so a full
/var or /data still warns) and on the full-metrics page (per-mount + worst
trend). Added hover tooltips defining CPU / Memory / Disk / Load.

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

plugins/host_agent/templates/widget_table.html

@@ -16,8 +16,8 @@
       {% if r.mem_used_pct is not none %}
       <span title="Memory"><span style="color:var(--text-dim);">mem</span> {{ "%.0f"|format(r.mem_used_pct) }}%</span>
       {% endif %}
-      {% if r.disk_worst is not none %}
-      <span title="Worst disk mount"><span style="color:var(--text-dim);">disk</span> {{ "%.0f"|format(r.disk_worst) }}%</span>
+      {% if r.disk_root is not none %}
+      <span title="Root filesystem (/) usage"><span style="color:var(--text-dim);">disk /</span> {{ "%.0f"|format(r.disk_root) }}%</span>
       {% endif %}
       {% if r.load_1m is not none %}
       <span title="Load average 1m"><span style="color:var(--text-dim);">load</span> {{ "%.2f"|format(r.load_1m) }}</span>