docs: document i18n conventions for agents

AGENTS.md

@@ -15,6 +15,23 @@
 - Prefer composable primitives (signals, hooks, utilities) over deep inheritance or implicit global state.
 - When adding platform integrations (SSE, IPC, SDK), isolate them in thin adapters that surface typed events/actions.
 
+## Multi-Language Support (i18n)
+
+The UI uses a small custom i18n layer (no ICU/messageformat). When building features, never hardcode user-visible strings.
+
+- **Runtime API:** use `useI18n()` in components (`const { t } = useI18n();`) and `tGlobal(...)` in stores/non-component code.
+  - Implementation: `packages/ui/src/lib/i18n/index.tsx`
+- **Where messages live:** `packages/ui/src/lib/i18n/messages/<locale>/` as TypeScript objects (`"flat.dot.keys": "string"`).
+  - Each locale has an `index.ts` that merges message parts; duplicate keys throw at build time.
+  - Merge helper: `packages/ui/src/lib/i18n/messages/merge.ts`
+- **Adding a new string:** add it to the appropriate `.../messages/en/*.ts` part file, then add the same key to each other locale’s corresponding file.
+  - Missing translations fall back to English (and finally to the key), so gaps can be easy to miss.
+- **Interpolation:** placeholders are simple `{name}` replacements (word characters only). Avoid placeholders like `{file-name}`.
+- **Pluralization:** handle manually via separate keys like `something.one` / `something.other` and choose in code.
+- **Adding a new language:** add a new `messages/<locale>/` folder + `index.ts`, register it in `packages/ui/src/lib/i18n/index.tsx`, and add it to the language picker in `packages/ui/src/components/folder-selection-view.tsx`.
+- **Locale persistence:** the selected locale is stored in app preferences (`locale`) and persisted via the server config (default `~/.config/codenomad/config.json`).
+- **Avoid English-only paths:** do not import `enMessages` directly in feature code; always go through `t(...)` so locale changes apply.
+
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