docs: require i18n review for new features

[tyreseluo]

What changed

• added a new AGENTS.md rule requiring i18n checks during new feature design and implementation
• explicitly calls out translation key coverage and UI text alignment review

Why

New features can ship with missing translations or inconsistent layout across languages if i18n review is skipped. This documents the expectation earlier in the workflow so it is checked before code lands.

Impact

• improves consistency for future feature work
• reduces the chance of untranslated or misaligned UI text reaching users

Validation

• reviewed the AGENTS.md diff to confirm the PR only adds the new project rule