Enable usage with GitHub PATs

[julian-michelmann]

Description
Currently, the action requires a GitHub App for authentication. While best practice and secure, this setup can be verbose and harder for a quick setup.

Change

• Allow configuration using a GitHub Personal Access Token (PAT) as an alternative.
• Keep GitHub App support as the recommended method.

Why

• Lowers the entry barrier for testing and smaller projects.
• Gives users flexibility in how they authenticate.

Tasks

• Add support for GitHub PAT authentication.
• Update documentation with usage examples.