feat(host_agent): a temperature sensor is judged by how it moves against the CPU (#5783)

A board's spare thermistor inputs report whatever an open input reads, and
the value rule from #5759 only catches the impossible ones. Once an hour,
sensors.refresh judges each sensor against the host's CPU (Package id N,
Tctl/Tdie/TccdN, PECI Agent N) over a week of hourly averages. A sensor is
ruled out if it falls as the CPU warms (r <= -0.5) or holds within 0.5 °C
while the CPU moved 10 °C or more. A judgment needs at least 72 shared hours.

The host temperature leaves judged sensors out at ingest. The host page
lists them under "Not counted", with each reason given once.

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

docs/plugins/host-agent-design.md

@@ -262,6 +262,15 @@ A post may carry `events`, said once each, with the next post that gets through:
 
 Steward turns them into the host's downtime (`host_downtime`): **reboot** (a stop, then a boot after it), **agent_stop** (the host stayed up), **unclean** (a boot after the agent was last heard that nobody announced: power loss or a crash) and **lost_contact**. An announced stop explains the silence for `reboot_grace_seconds` (Settings → Plugins → host_agent, 10 minutes): `seconds_since_seen` reads 0 until then, and the host's other rules go quiet with it, as when it is offline. Past the grace, agent offline fires. The events live in memory, so a span the agent saw is lost if it restarts mid-outage; the boot time still reads the restart as unclean.
 
+### Which temperature sensors are thermometers (#5782, #5783)
+
+A board's spare thermistor inputs (a Nuvoton's SYSTIN, AUXTIN*) are often wired to nothing, and the kernel reports them anyway, at whatever the open input reads. The host's temperature (`temp_c_max`) leaves a sensor out when:
+
+- **its value is impossible** (at or below 0 °C, or at or above 125 °C; `counts_as_hottest`), or
+- **it doesn't behave like a thermometer.** Once an hour, `sensors.refresh` judges each sensor against the host's CPU (Intel `Package id N`, AMD `Tctl`/`Tdie`/`TccdN`, else `PECI Agent N`), using the last 7 days of hourly averages. It needs at least 72 shared hours and a CPU that moved at least 10 °C. A sensor is ruled out when it **falls as the CPU warms** (Pearson r ≤ −0.5, the shape of an open input reading the chip's own reference) or **holds within 0.5 °C** while the CPU moved. A wired board or drive sensor that barely follows the CPU (r near 0) stays in.
+
+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.
+
 ### Logging
 
 To stderr only (systemd captures to journal). No file logging, no log rotation. Levels: