Feature: Add upload_file MCP tool (V2 protocol, main-connection upload)

Implement the P2 upload_file tool to push a local file or directory from the
master to an online Windows host over the existing V2 file-transfer protocol.
The server drives FileBatchTransferWorkerV2 synchronously on the main
connection through a headless callback, reuses the list_files chain for the
overwrite pre-check, and is gated by McpFileTransfer=1 plus McpReadonly=0.

The client main connection never initialized the file-transfer module, so
g_status stayed 0 and RecvFileChunkV2 silently dropped every chunk, truncating
uploads to zero bytes. Add a once-per-process lazy InitFileUpload in the
COMMAND_SEND_FILE_V2 handler that mirrors the FileManager init; the destructor
deliberately does not Uninit so g_status remains 1 across reconnects.

Update the design doc to record the client-side change and correct the
upload_file description to state that sha256 is not returned (V2 has no
receiver-to-sender ACK; integrity is checked client-side and logged only).

Co-Authored-By: deepseek-v4-pro

server/2015Remote/McpServer.h

@@ -218,6 +218,11 @@ public:
                                   const std::string& localDir, const std::string& remotePath,
                                   bool overwrite);
 
+    // 登记上传会话（false = 该 host 已有文件会话或一次性挂起请求）。upload 走主连接、
+    // 工具线程同步驱动 FileBatchTransferWorkerV2，会话仅为单设备单传输互斥标记
+    // （无 fmSubCtx/streamSubCtx，ClearFileTransfer 对空指针安全）。
+    bool BeginFileUpload(uint64_t device_id);
+
     // TOKEN_DRIVE_LIST：识别 download_file，存文件管理器子链接并下发 CMD_DOWN_FILES_V2。
     // 返回 true=已接管（保持子链接，收尾由 ClearFileTransfer）；false=会话已清理（调用方 CancelIO）。
     bool OnDownloadDriveList(uint64_t device_id, context* fmSubCtx);
@@ -317,8 +322,8 @@ private:
 
     // ===== P6：文件传输会话（受 m_FileXferMutex 保护；单设备单传输）=====
     struct FileTransferSession {
-        std::string tool;                    // 恒为 "download_file"
-        std::string localDir;                // 本机保存目录（ANSI，结尾 '\'）
+        std::string tool;                    // "download_file" / "upload_file"
+        std::string localDir;                // download：本机保存目录（ANSI，结尾 '\'）
         std::string remotePath;              // 客户端远程路径（ANSI，发 CMD_DOWN_FILES_V2 用）
         bool        overwrite = false;       // 是否覆盖已存在文件
         context*    fmSubCtx     = nullptr;  // 文件管理器子链接（下发 CMD_DOWN_FILES_V2）