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/__init__.py

@@ -36,9 +36,11 @@ async def mcp_overview(db) -> list[dict]:
 def get_alert_metrics() -> dict[str, list[str]]:
     """Host metrics ingest records for alert rules (resource = the host name;
     disk_used_pct per mount, resource "<host>:<mount>"). seconds_since_seen
-    comes from the scheduler, not the agent: "agent offline" is > 180."""
+    comes from the scheduler, not the agent: "agent offline" is > 180.
+    clock_off_secs is how far the host's clock is from Steward's, either way
+    (agent ≥ 1.12.0): "clock drifting" is > 30."""
     return {"host_agent": [
         "cpu_pct", "mem_used_pct", "mem_available_bytes", "swap_used_bytes",
         "disk_used_pct_worst", "disk_used_pct", "load_1m", "load_5m", "load_15m",
-        "uptime_secs", "seconds_since_seen",
+        "uptime_secs", "seconds_since_seen", "clock_off_secs",
     ]}