update-translations: apply translation updates through core's updater #352

New tool saddle/update-translations: apply the translation updates WordPress offers, through core's language pack upgrader in the background runner.

• Admin tier, destructive, gated; the token binds the exact list previewed.
• No item list and no 10-item cap: WordPress applies language packs on its own in every background update run, and sites often have dozens.

Acceptance: a real language pack installed by the runner in tests, the gate matrix, the hard-line test extended.