decide() grants REPLACE_HOST for an owner whose served set is unknown, but
_replace_target_belongs_to_other_profile can then only prove ownership from
THIS home's gateway.pid record (owner.serves() is False when served_known is
False, so the cross-home served-set shortcut never fires). "Whichever home
launched it" is therefore true only for the serves-this-profile leg; a
served-unknown owner launched from another home is refused (exit 1, one
supervisor retry). Say so at the host_attach module doc, the decide()
REPLACE_HOST comment and the _host_attach_or_none comment, and qualify the
`--replace` remedy in _unknown_served_message the same way.
_refuse_message is reached for an owner known NOT to serve this profile, so
its "take the host over: --replace" line pointed at a path that lands on the
same refusal (decide() only returns REPLACE_HOST for an owner that serves us
or has an unknown served set). Drop it in favour of a note, and interpolate
MIGRATE_COMMAND like the sibling refusal in run.py instead of hard-coding the
command. Text/comment changes only; no behaviour change.
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