fix(install): stop advising a -d flag that does not exist

[Chemaclass]

🤔 Background

Related #1221

All three destination errors in install.sh ended with Choose another destination with -d. The script parses positional arguments only, so following that advice returns Invalid arguments — the one message whose job is to tell the reader how to recover pointed at a form the parser rejects.

💡 Changes

• Name the real interface in the not-a-directory, cannot-create and not-writable messages
• Test by running the suggested form rather than string-matching it; the previous tests asserted the diagnosis line only, which is how an invented flag survived
• Assert the advice on the run that actually prints it — a flag-shaped argument dies in the parser before any advice, so checking it there compares against an empty string and passes for free