feat(alerts): the host's temperature and throughput can drive alert rules (#5787)

temp_c_max was recorded on every ingest, but host_agent didn't offer it
to rules, so "host running hot" couldn't be watched. The catalog guard
didn't see the row(...) helper ingest builds its rows with. Now offered:
temp_c_max, temp_c per sensor, and network and disk throughput per host,
interface and disk. A new guard holds every literal row(...) name to the
catalog, except NOT_ALERTABLE, where each exclusion is given a reason.
temp_c_max reads in °C.

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

docs/plugins/host-agent-design.md

@@ -271,6 +271,8 @@ A board's spare thermistor inputs (a Nuvoton's SYSTIN, AUXTIN*) are often wired
 
 The verdicts are kept in memory and rebuilt at startup, so ingest and the host page read them without a query. The host page lists the sensors left out under "Not counted", each with its reason. A host with no CPU reading, or too little history, falls back to the value rule alone.
 
+`temp_c_max` can drive an alert rule (#5787), e.g. "Host running hot": all hosts, over 85 °C for 10 consecutive checks. So can each sensor's `temp_c`, and the per-interface and per-disk throughput. `tests/core/test_metric_catalog.py` holds every metric ingest writes to the alert catalog, except the few `NOT_ALERTABLE` names in `plugins/host_agent/__init__.py`.
+
 ### Logging
 
 To stderr only (systemd captures to journal). No file logging, no log rotation. Levels: