Added agentic translation review workflow (#27966)

no ref

Adds an advisory CI bot that posts a **draft GitHub review** on any PR
labelled `affects:i18n`. The bot reads the PR's changes to
`ghost/i18n/locales/**/*.json`, asks Claude Sonnet 4.6 (via direct
Anthropic API + tool-use structured output) to validate the translations
against the English source and `context.json`, and posts the result as a
pending review with:

- A **yes/no verdict** in the body — `✅ Looks good` or `⚠️ Has
questions`
- **Inline line comments** on each flagged translation
- A polite "thank you" note to the translator in their language

The review is always a **draft** — a maintainer must submit, dismiss, or
edit before merge. `GITHUB_TOKEN` cannot approve PRs by design, so the
bot can never auto-approve even if it tried.

Approach is adapted from
[`cathysarisky/i18n_tooling`](https://github.com/cathysarisky/i18n_tooling),
which was the manual CLI predecessor. Bringing it in-repo gives Ghost
ownership of the prompt and removes a dependency on a personal repo.