When ci-test-scope falls back to the 'unit' sentinel (#408 intent) and
ci-prepare-test-scope writes it verbatim, run-tests --files-from received
a bare 'unit' token that was not a filename, causing exit 2 with
"requested test file(s) not found: unit".
selectExplicitFiles() now recognises any SUITES member and delegates to
the existing selectFiles() resolver before the path-existence check,
reusing the suite expansion logic rather than reimplementing it.
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
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