feat(dashboard): phase A widget clarity — threshold colours, host links, tooltips

Milestone 72 phase A (clarity wins, no schema change):
- Add shared metric_style() macro in _macros.html: colours a numeric metric
  amber (>=warn) / red (>=crit) on the value ITSELF, not just the status dot.
  Defaults 80/90 for percentage gauges; load /core uses warn 80 / crit 100.
  Applied to CPU/mem/disk across host_agent panel + fleet widget + the unified
  hosts-overview widget.
- Link every host reference to the host hub (/hosts/<id>) in ping, dns,
  hosts-overview, host_agent fleet, and uptime widgets; status widget entries
  link to their own detail_url. All guarded on session.user_id so the public
  share view degrades to plain text (the bare href carries no share token).
- Fix truncation: title= tooltips on all names that ellipsis, plus a hover
  underline affordance on the now-clickable ping-name links.

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

plugins/host_agent/templates/panel.html

@@ -1,4 +1,5 @@
 {# Per-host agent panel — embedded into /hosts/<id> via HTMX. Self-contained. #}
+{% from "_macros.html" import metric_style %}
 {% set scope = "steward:target:" ~ target.id if target else "" %}
 <div class="card">
   <div style="display:flex;align-items:baseline;justify-content:space-between;margin-bottom:0.75rem;gap:1rem;flex-wrap:wrap;">
@@ -21,19 +22,20 @@
   <div style="display:flex;gap:1.5rem;flex-wrap:wrap;margin-bottom:0.6rem;">
     <div title="Average CPU utilization across all cores">
       <div style="{{ lbl }}">CPU</div>
-      <div style="font-weight:600;">{{ '%.0f%%'|format(cpu) if cpu is not none else '—' }}</div>
+      <div style="font-weight:600;{{ metric_style(cpu) }}">{{ '%.0f%%'|format(cpu) if cpu is not none else '—' }}</div>
       {{ sparks.cpu | safe }}</div>
     <div title="RAM in use (total minus available; cache/buffers count as free)">
       <div style="{{ lbl }}">Memory</div>
-      <div style="font-weight:600;">{{ '%.0f%%'|format(mem) if mem is not none else '—' }}</div>
+      <div style="font-weight:600;{{ metric_style(mem) }}">{{ '%.0f%%'|format(mem) if mem is not none else '—' }}</div>
       {{ sparks.mem | safe }}</div>
     <div title="Root filesystem (/) usage — see Full metrics for every mount">
       <div style="{{ lbl }}">Disk /</div>
-      <div style="font-weight:600;">{{ '%.0f%%'|format(disk_root) if disk_root is not none else '—' }}</div>
+      <div style="font-weight:600;{{ metric_style(disk_root) }}">{{ '%.0f%%'|format(disk_root) if disk_root is not none else '—' }}</div>
       {{ sparks.disk | safe }}</div>
+    {# Load /core: 100% = run queue matches CPU capacity, so warn 80 / crit 100. #}
     <div title="1-minute load average ÷ {{ cores or '?' }} CPU cores (100% = run queue matches capacity). Raw 1m load: {{ '%.2f'|format(load1) if load1 is not none else '—' }}.">
       <div style="{{ lbl }}">Load /core</div>
-      <div style="font-weight:600;">{{ '%d%%'|format(load_per_core) if load_per_core is not none else ('%.2f'|format(load1) if load1 is not none else '—') }}</div>
+      <div style="font-weight:600;{{ metric_style(load_per_core, warn=80, crit=100) }}">{{ '%d%%'|format(load_per_core) if load_per_core is not none else ('%.2f'|format(load1) if load1 is not none else '—') }}</div>
       {{ sparks.load | safe }}</div>
   </div>
   {% if psi.cpu is not none or psi.mem is not none or psi.io is not none %}