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/agent.py

@@ -20,7 +20,7 @@ from collections import deque
 from concurrent.futures import ThreadPoolExecutor, wait as wait_futures
 from datetime import datetime, timezone
 
-AGENT_VERSION = "1.11.0"
+AGENT_VERSION = "1.12.0"
 
 # Default path to the local Docker Engine socket. Overridable via the
 # `docker_socket` config key; collection is silently skipped if it's absent or
@@ -1320,10 +1320,13 @@ def build_sample(mounts: list[str], state: dict,
 
 
 def build_payload(samples: list[dict], hostname: str, metadata: dict) -> dict:
+    # sent_at is this host's clock at the moment of sending: Steward compares
+    # it with its own on arrival to say how far apart the two clocks are.
     return {
         "agent_version": AGENT_VERSION,
         "hostname": hostname,
         "metadata": metadata,
+        "sent_at": datetime.now(timezone.utc).isoformat(),
         "samples": samples,
     }