Create copilot-setup-steps.yml

[minestarks]

This sets up the environment for Copilot Coding Agent to work in.

[swernli]

I wonder if there is a way to have this build off of the devcontainer definition we already have in the repo (https://github.com/microsoft/qsharp/blob/main/.devcontainer/devcontainer.json). I try to keep that up to date for internal or external contributors, and it seems like it could be opportunity to take advantage of existing setup.

[minestarks]

@swernli I like the idea, but I'll throw the question back at you - can we use that devcontainer for ci.yml too? I tried to align this workflow with ci.yml since I consider that the source of truth but maybe there's an opportunity for all of it to be aligned?

[minestarks]

@swernli browsed our devcontainer definition, and options to bring it into CI, like https://github.com/devcontainers/ci, and I think what I have is the better fit. Even if we could run the devcontainer in this workflow, I don't really see a way to get Copilot to run in the devcontainer.

We want to make sure Copilot is working within the exact "official" environment to maximize the chances of getting a clean build before publishing PRs.

It's also useful to run ./build.py once before Copilot gets going so that things like TypeScript header files get generated, and Copilot can proceed to work on smaller subcomponents and get reliable feedback from build commands. In my experiments, this setup just seemed to reduce confusion for Copilot.