chore: add GitHub Actions workflow to sync all upstream branches from xixu-me/Xget

[Copilot]

Summary

• Adds a scheduled workflow that mirrors every branch from the upstream repo (xixu-me/Xget) into this fork by force-pushing each branch to origin.
• Keeps the fork in sync without manual intervention; runs daily and is also triggerable on demand via workflow_dispatch.

New workflow: .github/workflows/upstream-sync.yml

• Adds xixu-me/Xget as upstream remote
• git fetch upstream to pull all remote refs
• Iterates over upstream/* branches and force-pushes each to origin
• Runs daily at 00:00 UTC; concurrency guard prevents overlapping runs

Testing

• npm run lint
• npm run format:check
• npm run test:run
• npm run type-check

Checklist

• I kept the change focused and avoided unrelated edits
• I added or updated tests when behavior changed
• I updated documentation when user-facing behavior changed
• I removed or redacted secrets, tokens, and private data from examples

Notes for reviewers

• Risk or rollout notes: Uses --force push — any local commits on synced branches will be overwritten. Branches exclusive to this fork are unaffected (only branches that exist in upstream are touched).