fix: prioritize GITEA_SERVER_URL over GITHUB_SERVER_URL for custom Gitea instances

- Modified config.ts to check GITEA_SERVER_URL first, then fall back to GITHUB_SERVER_URL
- Fixes issue where container-based Gitea instances generate internal URLs in comments
- Added comprehensive test suite for URL configuration scenarios
- Updated README.md with configuration documentation and examples
- Added example workflow file for custom URL setup
- Fixed TypeScript configuration to support Node.js globals

Resolves issue #4: Enable GITEA_SERVER_URL environment variable override

src/github/api/config.ts

@@ -7,8 +7,25 @@ function deriveApiUrl(serverUrl: string): string {
   return `${serverUrl}/api/v1`;
 }
 
-export const GITEA_SERVER_URL =
-  process.env.GITHUB_SERVER_URL || "https://github.com";
+// Get the appropriate server URL, prioritizing GITEA_SERVER_URL for custom Gitea instances
+function getServerUrl(): string {
+  // First check for GITEA_SERVER_URL (can be set by user)
+  const giteaServerUrl = process.env.GITEA_SERVER_URL;
+  if (giteaServerUrl && giteaServerUrl !== "") {
+    return giteaServerUrl;
+  }
+
+  // Fall back to GITHUB_SERVER_URL (set by Gitea/GitHub Actions environment)
+  const githubServerUrl = process.env.GITHUB_SERVER_URL;
+  if (githubServerUrl && githubServerUrl !== "") {
+    return githubServerUrl;
+  }
+
+  // Default fallback
+  return "https://github.com";
+}
+
+export const GITEA_SERVER_URL = getServerUrl();
 
 export const GITEA_API_URL =
   process.env.GITEA_API_URL || deriveApiUrl(GITEA_SERVER_URL);