feat(i18n): add i18n-localization skill with checker script

.agents/skills/i18n-localization/SKILL.md

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+---
+name: i18n-localization
+description: "Internationalization and localization patterns. Detecting hardcoded strings, managing translations, locale files, RTL support."
+---
+
+# i18n & Localization
+
+> Internationalization (i18n) and Localization (L10n) best practices.
+
+---
+
+## 1. Core Concepts
+
+| Term | Meaning |
+|------|---------|
+| **i18n** | Internationalization - making app translatable |
+| **L10n** | Localization - actual translations |
+| **Locale** | Language + Region (en-US, tr-TR) |
+| **RTL** | Right-to-left languages (Arabic, Hebrew) |
+
+---
+
+## 2. When to Use i18n
+
+| Project Type | i18n Needed? |
+|--------------|--------------|
+| Public web app | ✅ Yes |
+| SaaS product | ✅ Yes |
+| Internal tool | ⚠️ Maybe |
+| Single-region app | ⚠️ Consider future |
+| Personal project | ❌ Optional |
+
+---
+
+## 3. Implementation Patterns
+
+### Vue (i18next-vue)
+
+```vue
+<script setup lang="ts">
+import { computed } from 'vue'
+import { useTranslation } from 'i18next-vue'
+
+const { t } = useTranslation()
+
+const title = computed(() => t('welcome.title'))
+
+function getRequiredRule() {
+  return { required: true, message: t('form.required') }
+}
+</script>
+
+<template>
+  <h1>{{ $t('welcome.title') }}</h1>
+  <p>{{ title }}</p>
+  <p>{{ getRequiredRule().message }}</p>
+</template>
+```
+
+### Python (gettext)
+
+```python
+from gettext import gettext as _
+
+print(_("Welcome to our app"))
+```
+
+---
+
+## 4. File Structure
+
+Frontend locales:
+
+```
+locales/
+├── en-US.json
+├── zh-CN.json
+├── ar-SA.json        # RTL
+```
+---
+
+## 5. Terminology
+
+See [terminology.md](references/terminology.md).
+
+- Keep UI naming consistent (应用/Application, 控制台/Console)
+- Do not translate product names and code identifiers
+
+---
+
+## 6. Best Practices
+
+See [best-practices.md](references/best-practices.md).
+
+- Use translation keys and namespaces
+- Avoid string concatenation; use placeholders/plurals
+- Format dates/numbers by locale; allow text expansion/RTL
+
+---
+
+## 7. Checklist
+
+- [ ] No hardcoded user-facing strings
+- [ ] Locale files exist for all supported languages
+- [ ] Fallback language is configured
+- [ ] i18n checker script passes
+- [ ] Terminology follows the project glossary
+
+---
+
+## Script
+
+| Script | Purpose | Command |
+|--------|---------|---------|
+| `scripts/i18n_checker.py` | Detect hardcoded strings & missing translations | `python scripts/i18n_checker.py <project_path>` |
+
+## When to Use
+Use this skill to audit i18n usage and locale completeness.