Add VSCode/Copilot support

[cweagans]

Here's how I set it up for myself (I had to edit a bunch of stuff to remove Claude/Cursor references):

https://github.com/cweagans/dotfiles/blob/main/dot_config/sd/executable_setup-copilot

You can take off the little preamble bit at the top and run that directly in whatever project you want to set it up in. Copilot isn't allowed to read files outside of the current project, so I just added everything in the .github directory. The file naming is important -- .instructions.md get loaded automatically. .github/prompts/.prompt.md shows up as a slash command in the copilot chat as long as the user has "chat.promptFiles": true in their vscode config.