GitHub Bot support for automated PR reviews

[canefe]

Feature description

The current implementation targets GitLab Merge Requests.

Add support for a GitHub bot that triggers automated reviews on pull requests. This will require abstracting workflow logic that is currently specific to GitLab so that the core review engine can work with GitHub events.

Prerequisites

• Refactor the existing workflow to remove direct coupling to GitLab Merge Requests. #6

Done when

• GitHub webhook handling for pull request events added.

• Bot posts review comments on GitHub PRs using the GitHub API.

• Configurable trigger commands (e.g., /reviewbot review).

• Tests validating GitHub PR event handling.