HA add-on: 'auto-configure' stops short — HA's own MQTT integration must still be added by hand

[muness]

Verified live on the owner's HA with add-on 3.7.0-alpha.5. The UHC side of MQTT now auto-wires perfectly:

{"configured":true,"enabled":true,"running":true,"connection":"connected",
 "host":"core-mosquitto","port":1883,"source":"environment"}

UHC is connected to the Mosquitto broker and publishing discovery messages. But no entities appear, because Home Assistant's own MQTT integration is not configured (Settings → Devices & services → MQTT shows "No entries"), and MQTT was NOT offered under Discovered. Discovery messages sit in the broker with nothing consuming them.

So the story we tell ("install the add-on and zones appear") is still not true end to end: the user must add the MQTT integration in HA themselves.

Wanted

1. Add-on DOCS.md + CHANGELOG: state plainly that HA's MQTT integration must be configured, with the exact click path, and that it normally auto-fills the Mosquitto broker.
2. Better: detect it. The add-on has the Supervisor token, so it can query HA for whether an mqtt config entry exists and surface a clear, actionable message in UHC's Settings when we are publishing into a broker no one is consuming — the same honesty principle as MQTT status reports running=true while the broker is unreachable #607, one level up.
3. Consider whether the add-on can trigger HA's MQTT discovery flow rather than only telling the user about it.

Related: #605 (auto-wire), #607 (honest broker connectivity).