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/index.ts

@@ -6,6 +6,7 @@ export type {
   OhMyOpenCodeConfig,
   AgentOverrideConfig,
   AgentOverrides,
+  I18nConfig,
   McpName,
   AgentName,
   HookName,