The clone publication step did `mv <staged>/tree "$INSTALL_DIR"`. When
INSTALL_DIR already existed as a non-git directory, mv moved the
checkout INSIDE it as INSTALL_DIR/tree and the stage reported success,
leaving later stages to read pm/lock.json from a directory that holds
the user's files instead of a checkout. An existing file made mv fail
with a generic "cannot publish" error.
Before staging the clone, refuse a destination that exists and is not a
Hermes checkout (non-empty dir, file, or symlink) and say what to do; an
empty directory is taken over so the checkout lands AT the path.
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