dotnet/runtime ? reverse-engineered prompt

I want this repo set up so I can build the .NET runtime from source on my machine without a bunch of manual guesswork. Please inspect the repo, figure out the normal developer workflow, and get it into a state where I can compile the runtime and libraries, run a reasonable smoke test, and understand how to use the locally built dotnet host afterward.

If anything is missing on my machine, add clear setup steps and make the build commands easy to run on my platform. I would also like a short contributor friendly guide in plain English that explains what this repo actually contains, how to do a clean build, how to rerun tests, and where a newcomer should look first for help wanted or good first issues. If there are existing scripts and docs for this, use those instead of inventing a custom flow.

Look up current docs online if you need to, but keep the result practical and ready to use locally.