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/database/schema.rs

@@ -93,7 +93,9 @@ impl Database {
             enabled_codex BOOLEAN NOT NULL DEFAULT 0,
             enabled_gemini BOOLEAN NOT NULL DEFAULT 0,
             enabled_opencode BOOLEAN NOT NULL DEFAULT 0,
-            installed_at INTEGER NOT NULL DEFAULT 0
+            installed_at INTEGER NOT NULL DEFAULT 0,
+            content_hash TEXT,
+            updated_at INTEGER NOT NULL DEFAULT 0
         )",
             [],
         )
@@ -393,6 +395,11 @@ impl Database {
                         Self::migrate_v5_to_v6(conn)?;
                         Self::set_user_version(conn, 6)?;
                     }
+                    6 => {
+                        log::info!("迁移数据库从 v6 到 v7（Skills 更新检测支持）");
+                        Self::migrate_v6_to_v7(conn)?;
+                        Self::set_user_version(conn, 7)?;
+                    }
                     _ => {
                         return Err(AppError::Database(format!(
                             "未知的数据库版本 {version}，无法迁移到 {SCHEMA_VERSION}"
@@ -1045,6 +1052,14 @@ impl Database {
         Ok(())
     }
 
+    /// v6 -> v7: Skills 更新检测支持（content_hash + updated_at）
+    fn migrate_v6_to_v7(conn: &Connection) -> Result<(), AppError> {
+        Self::add_column_if_missing(conn, "skills", "content_hash", "TEXT")?;
+        Self::add_column_if_missing(conn, "skills", "updated_at", "INTEGER NOT NULL DEFAULT 0")?;
+        log::info!("v6 -> v7 迁移完成：已添加 content_hash 和 updated_at 列");
+        Ok(())
+    }
+
     /// 插入默认模型定价数据
     /// 格式: (model_id, display_name, input, output, cache_read, cache_creation)
     /// 注意: model_id 使用短横线格式（如 claude-haiku-4-5），与 API 返回的模型名称标准化后一致