Customize the development environment for Copilot Coding Agent

[dylan-smith]

Description

I've been experimenting with having the Copilot Coding Agent implement some of the outstanding issues on this repo, and in general it's been doing a pretty impressive job. Some examples here:

• #1354
• #1352
• #1351
• #1349

But we can make it even better by making some small tweaks to the repo!

It consistently tries to change the .Net SDK version in it's PR's to match the version installed in the Copilot environment. Which is undesirable and I have to keep telling it in the PRs to stop doing that (for example).

A simple way to fix this would be to customize the copilot development environment by adding a copilot-setup-steps.yml workflow to explicitly tell copilot how it should setup it's development environment to match the one that we use.

[timrogers]

As you're seeing, Copilot isn't amazing at making Copilot setup steps files because they didn't exist when the modal was trained 😰

We need to find a way to make this better - probably with some special prompting with someone asks for custom setup steps.