export/import round-trip fails on an empty GOBIN; remove prints a bare EOF on closed stdin

[nao1215]

Two gup UX issues found while writing atago E2E coverage against a fake GOBIN. Neither is a crash; both are message/consistency problems.

1. export then import is not a round-trip on an empty GOBIN

mkdir -p /tmp/emptybin /tmp/xdg
GOBIN=/tmp/emptybin XDG_CONFIG_HOME=/tmp/xdg gup export           # WARN + exit 0, writes {"schema_version":1,"packages":[]}
GOBIN=/tmp/emptybin XDG_CONFIG_HOME=/tmp/xdg gup import --dry-run # exit 1: "unable to import package: no package information"

export succeeds (exit 0) and writes a valid, empty gup.json, but importing that same file fails with a generic error. Expected: either import accepts an empty configuration ("nothing to import", exit 0), or export of an empty configuration is itself an error — the two sides should agree. The error also does not name the file it tried to read.

2. remove on stdin EOF prints a bare "gup:ERROR: EOF"

GOBIN=<fake-gobin> gup remove sometool < /dev/null
# "gup:CHECK: remove ...? [Y/n] gup:ERROR: EOF", exit 1 (nothing removed, which is safe)

The confirmation prompt surfaces the raw bufio EOF. Expected: a message with context (e.g. "confirmation aborted: stdin closed") or a non-interactive default of "No". Behavior is safe; only the message is unclear.

Found while writing atago E2E coverage.