Add durable git execution worker for production deployments

[aniruddhaadak80]

Summary

Move git mutations out of ephemeral serverless storage into a durable worker or VM-backed runtime.

Problem

The app can create and merge repos on disk locally, but Vercel serverless storage is ephemeral. Production repo operations need a durable execution path.

Scope

• Design a worker execution model for repo create/branch/commit/merge actions.
• Keep the current local runtime path for development.
• Add a job interface between the Next.js app and the worker.
• Document deployment assumptions.

Acceptance Criteria

• Git actions no longer depend on local serverless disk in production.
• Local development flow still works.
• README and deployment docs explain the architecture.