Provide a way for extensions to give chat context

[alexr00]

GitHub Pull Requests wants to provide context to chat when there's a PR currently checked out. Ideally, there's be some VS Code extension API or some Copilot Chat extension API that allows extensions to register context providers.

[alexr00]

It would also be useful for extensions to fire an event when the context has changed.

[roblourens]

Are you looking for an extension API to contribute the <repoContext> that's added to the agent prompt right now? Is there something that you can do that the copilot extension can't do?

[alexr00]

I think so. We want to include info about whether the current branch is a PR. We have the convoluted and backwards thing now:

• PR extension exposes API to get current state
• Copilot Chat finds the PR extension, calls this state API, and adds it to the prompt.

Copilot Chat shouldn't really know about the PR extension. Instead, I would expect VS Code extension API or Copilot Chat extension API that has something like this:

interface PromptContextProvider {
...
}

registerPromptContextProvider(provider: PromptContextProvider)

This would be better as VS Code extension API since extension API requires that the extensions be in the same extension host.

[roblourens]

Yeah, I think we want something like that. I probably won't get to it in July because I'll be out for part of it. I'm hoping that what we have now is stable at least, just ugly.