Feature: Multi-repository mode

In my workflow, I often need to introduce changes in one repository while simultaneously referencing interfaces, types, or contracts defined in another repository.

Currently, Mux can operate only within a single repository context. When it needs to reference another repository, it falls back to fetching files via the GitHub API (e.g. using curl). This approach is slow, inefficient, and breaks the flow, especially for larger repositories or frequent cross-repo lookups.