fix: add Windows support with entry_points and os.replace

- Replace os.rename() with os.replace() for atomic file operations
  on Windows (os.rename fails if destination exists on Windows)
- Add entry_points console_scripts for proper .exe generation on Windows
- Create github_backup/cli.py with main() entry point
- Add github_backup/__main__.py for python -m github_backup support
- Keep bin/github-backup as thin wrapper for backwards compatibility

Closes #112

github_backup/__main__.py

@@ -0,0 +1,13 @@
+"""Allow running as: python -m github_backup"""
+
+import sys
+
+from github_backup.cli import main
+from github_backup.github_backup import logger
+
+if __name__ == "__main__":
+    try:
+        main()
+    except Exception as e:
+        logger.error(str(e))
+        sys.exit(1)