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>

steward/core/alerts.py

@@ -81,9 +81,10 @@ def agents_away(hosts_away: Callable[[AsyncSession], Awaitable[set[str]]]) -> No
 
 def names_behind(source_module: str, metric_name: str,
                  names: Callable[[AsyncSession, str], Awaitable[list[str]]]) -> None:
-    """A figure that counts things: `await names(session, resource)` names
-    the things it counts right now ("web_app (0 of 1 running)"), so the
-    alert about it can say which. The plugin recording the figure registers it."""
+    """A figure that counts things, or stands for one of several: `await
+    names(session, resource)` names them right now ("web_app (0 of 1 running)",
+    "Package id 0 at 86°C"), so the alert about it can say which. The plugin
+    recording the figure registers it."""
     _NAMED[(source_module, metric_name)] = names