copilot setup steps

[danmoseley]

Description

Copilot currently can't build when assigned issues, because its firewall won't allow it to download anything. We could open the firewall, but recommendation is instead to create a yml that sets up the environment so it is all set for copilot to run with.

Following instructions from https://docs.github.com/en/copilot/customizing-copilot/customizing-the-development-environment-for-copilot-coding-agent#preinstalling-tools-or-dependencies-in-copilots-environment

@radical or @RussKie is this sufficient to build the repo such that nothing else needs downloading from that point on?

cc @captainsafia

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[radical]

The build step should be good. But there might be more dependencies when running tests, like Playwright which gets downloaded when preparing for helix, or running the tests. It might be useful to also pass /p:PrepareForHelix=true.

[danmoseley]

added prepareforhelix.