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

tsconfig.json

@@ -1,7 +1,7 @@
 {
   "compilerOptions": {
     // Environment setup & latest features
-    "lib": ["ESNext"],
+    "lib": ["ESNext", "DOM"],
     "target": "ESNext",
     "module": "ESNext",
     "moduleDetection": "force",