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-tauri/src/commands/skill.rs

@@ -8,7 +8,7 @@ use crate::app_config::{AppType, InstalledSkill, UnmanagedSkill};
 use crate::error::format_skill_error;
 use crate::services::skill::{
     DiscoverableSkill, ImportSkillSelection, Skill, SkillBackupEntry, SkillRepo, SkillService,
-    SkillUninstallResult,
+    SkillUninstallResult, SkillUpdateInfo,
 };
 use crate::store::AppState;
 use std::sync::Arc;
@@ -134,6 +134,33 @@ pub async fn discover_available_skills(
         .map_err(|e| e.to_string())
 }
 
+/// 检查 Skills 更新
+#[tauri::command]
+pub async fn check_skill_updates(
+    service: State<'_, SkillServiceState>,
+    app_state: State<'_, AppState>,
+) -> Result<Vec<SkillUpdateInfo>, String> {
+    service
+        .0
+        .check_updates(&app_state.db)
+        .await
+        .map_err(|e| e.to_string())
+}
+
+/// 更新单个 Skill
+#[tauri::command]
+pub async fn update_skill(
+    id: String,
+    service: State<'_, SkillServiceState>,
+    app_state: State<'_, AppState>,
+) -> Result<InstalledSkill, String> {
+    service
+        .0
+        .update_skill(&app_state.db, &id)
+        .await
+        .map_err(|e| e.to_string())
+}
+
 // ========== 兼容旧 API 的命令 ==========
 
 /// 获取技能列表（兼容旧 API）