fix: ensure deliverables directory is writable by container user (#116)

Pre-create the deliverables directory with proper permissions on the
host before starting containers, and surface permission errors instead
of silently swallowing them in save_deliverable.

mcp-server/src/utils/file-operations.ts

@@ -29,7 +29,7 @@ export function saveDeliverableFile(targetDir: string, filename: string, content
   try {
     mkdirSync(deliverablesDir, { recursive: true });
   } catch {
-    // Directory might already exist, ignore
+    throw new Error(`Cannot create deliverables directory at ${deliverablesDir}`);
   }
 
   // Write file (atomic write - single operation)