refactor(routes): extract shared status-label mutation helper

[joryirving]

Problem: src/app/api/issues/status/route.ts:6 and src/app/api/issues/move/route.ts:7 each define an identical local isStatusLabel and duplicate auth/parse/validation for status/* label mutation.

Fix: extract a shared isStatusLabel + mutateStatusLabels helper (into @/lib/github or a small module). Effort: Low.

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Part of #498 (dispatch audit).