Ability to add an open (but not checked out) PR as context to chat

[bamurtaugh]

Similar to https://github.com/microsoft/vscode-copilot/issues/18906, the more I do with Coding Agent PRs in VS Code, the more I want to be able to switch between them freely without needing to fully check them out.

I think I'd essentially like a version of #activePullRequest that'd work with just having the PR view open in VS Code (not requiring a checkout), i.e. #openPullRequest, or the ability to drag the PR tab as context into the chat panel.

Perhaps this issue is duplicative of the Explore integrated chat view for Coding Agent session work @joshspicer and @mjbvz are driving (microsoft/vscode#255483), since both involve being able to communicate back to the Coding Agent session. But I think #openPullRequest functionality could be perhaps an alternative flow (I don't want to switch chat modes or open a new chat view, and I may not even want to actually chat with the Coding Agent; I just want to quickly learn or ask about a PR locally).

[bamurtaugh]

Example flow I'm currently encountering:

• Open a PR that the Coding Agent has been working on
• Review the PR description and changes
• Decide I'd like to iterate
  • I asked the Coding Agent: @copilot How could I test these changes? Add that info the PR description in a comment
• The Coding Agent completed another session on my PR
• The Coding Agent replies with a summary of its work
• I need to iterate further with it
• I can add another comment with @copilot to the PR, but it'd be nice to do some iteration in the chat panel
  • Better understand the changes Copilot just made
  • Iterate on ideas for further changes

I could check out the PR for this, but since that takes some extra time, and I may have multiple related PRs of interest, I'd love to be able to quickly move between them in chat, without having to check them out.

[bamurtaugh]

I think this could be handled by @githubpr:

Though it doesn't detect there are any comments on it (when there are - PR I'm looking at is: #7317)

This also feels tied to:

• able to reference comments from PR in chat vscode#253426
• @githubpr missing context about me and my activity #7328
  • It doesn't fully understand my PR
• Participants aren't always sticky in agent mode vscode#256531
  • @githubpr isn't sticky when chatting with it in Ask mode, so it feels a bit manual to have to re-add it every time to get more context
• Coding Agent: Ability for cloud button to send as a comment to existing PR #7336