Coding Agent: Ability for cloud button to send as a comment to existing PR

[bamurtaugh]

In the chat panel, I was analyzing a PR Copilot created and wanted to iterate on it.

Due to limitations in being able to reference comments in chat (microsoft/vscode#253426), my flow ended up being:

• I take a screenshot of the comment from the PR
• Attach screenshot to chat
• Use local agent mode to learn more about the PR + comment

I wanted to then send that back to the Coding Agent via the cloud button.

But when I used the cloud button, I realized it packaged the conversation into a new PR (rather than a new session in existing PR), which doesn't make sense when I'm trying to iterate on an existing PR: #7335.

I would've loved the cloud button to initiate a new Coding Agent session on my existing PR. As one idea, if I have a PR checked out and then send something to the Coding Agent via the cloud button, it could just add an @copilot comment to my PR with a summary of the change I want.

Related: #7332