feat: add skill update detection via SHA-256 content hashing

- Add content_hash and updated_at fields to skills table (DB migration v6→v7)
- Compute directory content hash on install/import/restore for version tracking
- Add check_updates command: downloads repos, compares hashes, returns update list
- Add update_skill command: backs up old files, re-downloads and replaces SSOT
- Backfill content_hash for existing skills on first update check
- Add "Check Updates" button and per-skill update badge/button in UnifiedSkillsPanel
- Add i18n keys for zh/en/ja

src/i18n/locales/en.json

@@ -1573,6 +1573,15 @@
     "installFailed": "Failed to install",
     "uninstallSuccess": "Skill {{name}} uninstalled",
     "uninstallFailed": "Failed to uninstall",
+    "update": "Update",
+    "updating": "Updating...",
+    "updateAvailable": "Update",
+    "updateSuccess": "Skill {{name}} updated to latest version",
+    "updateFailed": "Failed to update",
+    "checkUpdates": "Check Updates",
+    "checkingUpdates": "Checking...",
+    "noUpdates": "All skills are up to date",
+    "updatesFound": "{{count}} skill(s) have updates available",
     "error": {
       "skillNotFound": "Skill not found: {{directory}}",
       "missingRepoInfo": "Missing repository info (owner or name)",

src/i18n/locales/ja.json

@@ -1573,6 +1573,15 @@
     "installFailed": "インストールに失敗しました",
     "uninstallSuccess": "スキル {{name}} をアンインストールしました",
     "uninstallFailed": "アンインストールに失敗しました",
+    "update": "更新",
+    "updating": "更新中...",
+    "updateAvailable": "更新あり",
+    "updateSuccess": "スキル {{name}} を最新バージョンに更新しました",
+    "updateFailed": "更新に失敗しました",
+    "checkUpdates": "更新を確認",
+    "checkingUpdates": "確認中...",
+    "noUpdates": "すべてのスキルは最新です",
+    "updatesFound": "{{count}} 個のスキルに更新があります",
     "error": {
       "skillNotFound": "スキルが見つかりません: {{directory}}",
       "missingRepoInfo": "リポジトリ情報（owner または name）が不足しています",

src/i18n/locales/zh.json

@@ -1573,6 +1573,15 @@
     "installFailed": "安装失败",
     "uninstallSuccess": "技能 {{name}} 已卸载",
     "uninstallFailed": "卸载失败",
+    "update": "更新",
+    "updating": "更新中...",
+    "updateAvailable": "可更新",
+    "updateSuccess": "技能 {{name}} 已更新到最新版本",
+    "updateFailed": "更新失败",
+    "checkUpdates": "检查更新",
+    "checkingUpdates": "检查中...",
+    "noUpdates": "所有技能已是最新版本",
+    "updatesFound": "发现 {{count}} 个技能有可用更新",
     "error": {
       "skillNotFound": "技能不存在：{{directory}}",
       "missingRepoInfo": "缺少仓库信息（owner 或 name）",