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/lib/api/skills.ts

@@ -25,6 +25,8 @@ export interface InstalledSkill {
   readmeUrl?: string;
   apps: SkillApps;
   installedAt: number;
+  contentHash?: string;
+  updatedAt: number;
 }
 
 export interface SkillUninstallResult {
@@ -78,6 +80,14 @@ export interface Skill {
   repoBranch?: string;
 }
 
+/** Skill 更新信息 */
+export interface SkillUpdateInfo {
+  id: string;
+  name: string;
+  currentHash?: string;
+  remoteHash: string;
+}
+
 /** 仓库配置 */
 export interface SkillRepo {
   owner: string;
@@ -149,6 +159,16 @@ export const skillsApi = {
     return await invoke("discover_available_skills");
   },
 
+  /** 检查 Skills 更新 */
+  async checkUpdates(): Promise<SkillUpdateInfo[]> {
+    return await invoke("check_skill_updates");
+  },
+
+  /** 更新单个 Skill */
+  async updateSkill(id: string): Promise<InstalledSkill> {
+    return await invoke("update_skill", { id });
+  },
+
   // ========== 兼容旧 API ==========
 
   /** 获取技能列表（兼容旧 API） */