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>

tests/plugins/host_agent/test_expand_metrics.py

@@ -107,6 +107,13 @@ def test_stalled_mounts_are_counted_and_have_no_figures():
     assert ("disk_used_pct", "d3:/nfs/media") not in idx
 
 
+def test_the_heat_alert_names_the_sensor_that_set_it():
+    from plugins.host_agent.routes import hottest_sensor
+    temps = {"Package id 0": 86.0, "Composite": 40.0, "AUXTIN0": 110.0, "PCH_CHIP_CPU_MAX_TEMP": 0.0}
+    assert hottest_sensor("box", temps) == ["Package id 0 at 86°C"]
+    assert hottest_sensor("box", {"AUXTIN0": 110.0}) == []
+
+
 def test_a_wired_board_input_still_counts():
     """#5759: below 100 °C a board input is a real reading."""
     from plugins.host_agent.routes import counts_as_hottest