Codex/repo setup docs

[SeaCelo]

Summary

This PR sets up the repository for outside contributors before major code work starts. It improves clarity, reduces onboarding friction, and defines a simple contribution workflow. There are no functional code changes.

What this PR changes

1. Rewrites README.md to be shorter and easier for new users.
2. Clearly states this repo is downstream from OSeMOSYS/MUIO and maintained independently.
3. Explains current usage path:
   • Windows users can use the existing .exe workflow.
   • Mac users can use SeaCelo/MUIO-Mac for now.
4. States long-term goal: make MUIOGO platform independent so a separate Mac port is no longer needed.
5. Makes contribution flow explicit: open an issue first, then work from a feature branch.
6. Updates supporting docs and PR guidance for consistency.

Solves #5

@autibet