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

assets/oh-my-opencode.schema.json

@@ -5946,6 +5946,15 @@
       ],
       "additionalProperties": false
     },
+    "i18n": {
+      "type": "object",
+      "properties": {
+        "locale": {
+          "type": "string"
+        }
+      },
+      "additionalProperties": false
+    },
     "team_mode": {
       "type": "object",
       "properties": {