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

client/KernelManager.cpp

@@ -1379,6 +1379,13 @@ VOID CKernelManager::OnReceive(PBYTE szBuffer, ULONG ulLength)
     }
 
     case COMMAND_SEND_FILE_V2: {
+        // 主连接接收 V2 上传：确保文件传输模块已初始化（RecvFileChunkV2 依赖 g_status==1）。
+        // static 标志保证进程内只初始化一次，不随主连接重连反复 Init/Uninit。
+        static bool s_v2RecvInited = false;
+        if (!s_v2RecvInited) {
+            InitFileUpload({}, m_LoginMsg, m_LoginSignature, 64, 50, Logf);
+            s_v2RecvInited = true;
+        }
         // C2C/V2 文件接收（RecvFileChunkV2 内部会打印进度）
         int n = RecvFileChunkV2((char*)szBuffer, ulLength, m_conn,
             nullptr, m_hash, m_hmac, m_MyClientID);