mbed-tools should allow me to pass -u option to show unmounted devices

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I want mbed-tools detect to allow me to list unmounted devices

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able to pass -u option to show unmounted devices

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I'm using mbed-ls for now which has the -u option

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Start with the mbed-tools detect command and compare its device filtering with mbed-ls's -u behavior. Trace the detect CLI entry point and its existing tests, then verify that passing -u lists unmounted devices while the default output remains unchanged.

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