fix(prompts): write next-step suggestions in the run's UI locale (#7920)

The three follow-up suggestions under a completed turn are written by the
model, not picked from i18n files, so the only thing that decides their
language is the prompt. That prompt carried one soft rule — "write them in
the language the user is speaking" — printed directly under three English
examples, and nothing about the run's actual locale.

The locale rule lived only in `# UI locale override`, a section of the
cache-stable prompt head that `server.ts` deliberately drops on resume turns
to protect the upstream prompt cache. The keyed follow-up block is re-sent
every turn. So the rule was absent exactly on the turns that carry the
marker, and a zh-CN run closed with English suggestions — one of them a
near-copy of the prompt's own example.

Thread the run locale into `renderChatTurnHostProtocolInstructions` so the
per-turn block states its own language requirement, including the existing
brand/code/technical-identifier exception so `localStorage` and `CTA` still
survive. An English or absent locale renders byte-for-byte what it did
before. Also collapses the third copy of the zh-CN/zh-TW name mapping into
`prompts/ui-locale.ts`.

Fixes OPEND-2765