With the homeassistant plugin missing, send_message to a `homeassistant:` target failed with
"Unknown or unregistered plugin platform: homeassistant" and no way forward.
Both unknown-platform errors (target resolution and platform config) now go through one helper
that appends, for a platform that left core (a LEFT_CORE row's platform), the exact install
command for the active home (`hermes [-p <profile>] plugins install homeassistant`). Other unknown
names keep the bare message.
f7689f40a4