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>

plugins/host_agent/templates/_host_vitals.html

@@ -16,6 +16,7 @@
     <span class="problem-detail">
       {%- if not stale %}seen {{ reg.last_seen_at|ago }}{% endif %}
       {%- if reg.agent_version %}{{ " · " if not stale }}agent v{{ reg.agent_version }}{% endif -%}
+      {%- if clock and clock[0] %} · <span{% if clock[1] in ('warn', 'crit') %} class="tone-{{ clock[1] }}" title="Further from Steward's clock than Settings → Thresholds allows: check what keeps this host's time in sync"{% endif %}>{{ clock[0] }}</span>{% endif -%}
     </span>
   </div>
   <div class="stat-strip host-health-tiles" style="margin-bottom:0;">