feat(host_agent): the heat alert says which sensor set the temperature

"Host running hot" read the host's hottest sensor and didn't name it, so the
operator couldn't tell what hit 100 °C on VDNT-Util01. The alert email now
names the sensor and its reading, through the same names_behind a count uses.

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

plugins/host_agent/__init__.py

@@ -25,6 +25,10 @@ def setup(app: "Quart") -> None:
     from steward.hosts.service import agent_stale_after_seconds
     agent_heartbeat("host_agent", "seconds_since_seen", lambda: agent_stale_after_seconds(app.config))
     agent_fed("host_agent", lambda resource: resource.split(":", 1)[0])
+    # temp_c_max stands for one of many sensors: its alert says which.
+    from steward.core.alerts import names_behind
+    from .routes import name_hottest_sensor
+    names_behind("host_agent", "temp_c_max", name_hottest_sensor)
     # A reboot the agent announced is silence with a reason, for a while (#5756).
     from datetime import datetime, timezone
     from steward.core.alerts import agents_away