Refactor backup runner and improve sync behavior

[Frazzer951]

Summary

• refactor the project into a proper package and remove the legacy wrapper script
• move config handling and scheduling into Python, add safer auth handling, and support
• speed up syncs with API pagination tuning and bounded parallel fetches
• reduce log noise by suppressing unchanged repo logs while keeping Git fetch output for changed repos
• refresh the README and repository metadata to match the new runtime behavior

Validation

• All checks passed!
• Listing 'github_backup'...
• manual local backup runs against to verify change-only logging and repo sync behavior