Language Injection for Agent Prompts

Authoritative developer guide: docs/dev-guides/6-i18n-language-injection.md.

Prerequisites: Read docs/dev-guides/6-i18n-language-injection.md before changing Agent prompts, Agent routes, backend user-visible messages, or frontend i18n strings. If your work introduces new language injection patterns or conventions, update this file and related dev-guides accordingly.

Architecture