debug: add SFTP message logging to diagnose empty directory listing

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/terminal/sftp.gateway.ts

@@ -24,8 +24,10 @@ export class SftpGateway {
     client.on('message', async (raw: Buffer) => {
       try {
         const msg = JSON.parse(raw.toString());
+        this.logger.log(`[SFTP] Message: ${msg.type} sessionId=${msg.sessionId} path=${msg.path || ''}`);
         await this.handleMessage(client, msg);
       } catch (err: any) {
+        this.logger.error(`[SFTP] Error: ${err.message}`);
         this.send(client, { type: 'error', message: err.message });
       }
     });