Support Dual Authentication (GitHub App and Direct Token)

[piraz]

Enable the action to authenticate using either a direct GitHub Token (PAT/GITHUB_TOKEN) or a GitHub App. This simplifies onboarding for users who prefer token-based setups while maintaining the secure GitHub App flow.

Key Changes:

1. Inputs: Add github-token to action.yml and make app-id/private-key optional.
2. Precedence:
   • Priority 1: Use github-token (Direct Auth).
   • Priority 2: Fallback to app-id + private-key (App Auth).
3. Security: Mask the token (passed or generated) with core.setSecret.
4. Operations: Use the token for both GitHub API calls (Octokit) and Git push (remote URL).