Rearranged the code and added CLI that installs tools locally.

src/core/downloader.ts

@@ -0,0 +1,52 @@
+import { getMatchingAsset } from './matcher';
+import { PlatformInfo } from './platform';
+
+export interface ReleaseAsset {
+  name: string;
+  browser_download_url: string;
+}
+
+export interface ReleaseInfo {
+  tag_name: string;
+  assets: ReleaseAsset[];
+}
+
+export async function fetchLatestRelease(repository: string, token?: string): Promise<ReleaseInfo> {
+  const url = `https://api.github.com/repos/${repository}/releases/latest`;
+  const headers: Record<string, string> = {
+    'Accept': 'application/vnd.github.v3+json',
+    'User-Agent': 'setup-github-release-action'
+  };
+  if (token) {
+    headers['Authorization'] = `token ${token}`;
+  }
+
+  const response = await fetch(url, { headers });
+  if (!response.ok) {
+    const errorBody = await response.text();
+    throw new Error(`Failed to fetch latest release for ${repository}: ${response.statusText}. ${errorBody}`);
+  }
+
+  return await response.json() as ReleaseInfo;
+}
+
+export async function downloadAsset(url: string, destPath: string, token?: string): Promise<void> {
+  const headers: Record<string, string> = {
+    'User-Agent': 'setup-github-release-action'
+  };
+  if (token) {
+    headers['Authorization'] = `token ${token}`;
+  }
+
+  const response = await fetch(url, { headers });
+  if (!response.ok) {
+    throw new Error(`Failed to download asset: ${response.statusText}`);
+  }
+
+  const fs = await import('fs');
+  const { Readable } = await import('stream');
+  const { finished } = await import('stream/promises');
+
+  const fileStream = fs.createWriteStream(destPath);
+  await finished(Readable.fromWeb(response.body as any).pipe(fileStream));
+}