docs(optimize): surface --whitelist as a way to keep a DMG mounted

The whitelist machinery has always honoured mount paths and image paths,
so users could already prevent a long-lived disk image from being
offered for detach. The discoverability was the gap (#960): the detach
prompt did not hint at the escape hatch and the README mention was
generic.

Add a one-line hint under the detach candidate list that names
`mo optimize --whitelist` and points out that path patterns work, and
extend the README sentence with the same use case as a concrete example.