feat(i18n): add toast i18n with en/zh locale and plugin config support

- Add src/locales/ with en baseline and zh overrides (Partial<Record> fallback)
- Add src/shared/i18n.ts with initI18n/t/setLocale/getLocale (LANG env auto-detect)
- Add I18nConfigSchema with locale field to plugin config
- Internationalize 13 hardcoded strings in task-toast-manager
- Add 18 unit tests for i18n module
- Pin manager tests to en locale for determinism

src/config/schema/i18n.ts

@@ -0,0 +1,8 @@
+import { z } from "zod"
+
+export const I18nConfigSchema = z.object({
+  /** Override auto-detected locale (e.g. "en", "zh"). Falls back to LANG env var if not set. */
+  locale: z.string().optional(),
+})
+
+export type I18nConfig = z.infer<typeof I18nConfigSchema>