Fix: Use Node.js script for cross-platform file copying in build

- Replaces the rsync command in the build_package.sh script with a
  Node.js script (copy_files.cjs) to handle copying necessary files
  (e.g., .md, .json, .sb) during the build process.
- Addresses an issue where the build would fail on systems
  that do not have rsync installed (e.g., some Windows environments or
  minimal Linux distributions) by using a Node.js script, providing a
  cross-platform solution as Node.js is already a project dependency.
- Updates the ESLint configuration to correctly lint .cjs files as
  CommonJS modules.

Fixes https://github.com/google-gemini/gemini-cli/issues/387

scripts/build_package.sh

@@ -26,8 +26,8 @@ fi
 # build typescript files
 tsc --build
 
-# copy .{md,json} files  (replace -q with -i to see itemized changes)
-rsync -aq --delete --include='*.md' --include='*.json' --include='*.sb' --include='*/' --exclude='*' ./src/ ./dist/src/
+# copy .{md,json} files
+node ../../scripts/copy_files.cjs
 
 # touch dist/.last_build
 touch dist/.last_build

scripts/copy_files.cjs

@@ -0,0 +1,49 @@
+#!/usr/bin/env node
+// Copyright 2025 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+const fs = require('fs');
+const path = require('path');
+
+const sourceDir = path.join('src');
+const targetDir = path.join('dist', 'src');
+
+const extensionsToCopy = ['.md', '.json', '.sb'];
+
+function copyFilesRecursive(source, target) {
+  if (!fs.existsSync(target)) {
+    fs.mkdirSync(target, { recursive: true });
+  }
+
+  const items = fs.readdirSync(source, { withFileTypes: true });
+
+  for (const item of items) {
+    const sourcePath = path.join(source, item.name);
+    const targetPath = path.join(target, item.name);
+
+    if (item.isDirectory()) {
+      copyFilesRecursive(sourcePath, targetPath);
+    } else if (extensionsToCopy.includes(path.extname(item.name))) {
+      fs.copyFileSync(sourcePath, targetPath);
+    }
+  }
+}
+
+if (!fs.existsSync(sourceDir)) {
+  console.error(`Source directory ${sourceDir} not found.`);
+  process.exit(1);
+}
+
+copyFilesRecursive(sourceDir, targetDir);
+console.log('Successfully copied files.');