labels <n> remove used to silently skip an unknown label name (lookup_id
returning None just did nothing) and discard each remove call's Result,
so a typo or a forge-rejected removal (403, insufficient permission)
both exited 0 with the label untouched.
Resolve names through the same resolve_ids labels add already uses, so
an unknown name is refused before anything is removed. After removing,
re-fetch and diff against what was requested — following assign.rs's
same before/after check — and bail! naming any label still present.
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