feat(host_agent): how far each host's clock is from Steward's (milestone 490)

- Agent 1.12.0 stamps each post with sent_at; ingest compares it with the
  moment the post arrived: clock_offset_s / clock_checked_at on the
  registration (host_agent_003), and a clock_off_secs metric (either way)
  that alert rules can watch. Replaces the old skew log, which measured the
  newest sample and was off by up to an interval.
- Settings -> Thresholds: Clock drift, 30 s warn / 5 min critical.
- Host page: 'clock 0.3s ahead' in the vitals line, toned past the warning.
- Hosts list: names hosts whose clock is off; when every reporting host is
  off by about the same amount, says Steward's own clock is the likely one.

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

steward/templates/hosts/list.html

@@ -22,6 +22,14 @@
         {% endif %}
     </div>
 </div>
+{% if clock %}
+{#- Milestone 490: a host whose clock drifts, or Steward's own when every host disagrees with it. #}
+<p class="page-verdict tone-warn">
+  {%- if clock.steward %}Every host's clock is about {{ clock.typical|duration }} off from Steward's: Steward's own clock is the likely one that is wrong. Check what keeps the time in sync on the machine Steward runs on.
+  {%- else %}Clock off from Steward's on {% for name, secs in clock.hosts %}<a href="/hosts/{{ host_ids[name] }}">{{ name }}</a> by {{ secs|duration }}{{ ", " if not loop.last }}{% endfor %}. Check what keeps the time in sync there.
+  {%- endif -%}
+</p>
+{% endif %}
 {% if total %}
 <form class="filter-bar" method="get" action="/hosts/" role="search"
       hx-target="#hosts-table" hx-push-url="true" hx-include="closest form">