[Feature]: Comment on/reply to specific agent response blocks.

[filipgutica]

I often quote something an agent said so I can ask a follow-up, clarify it, or push on one specific point.

Can we add the ability to comment on one or more specific agent response blocks and attach it as a "comment" in the prompt composer. Similar to how you would "comment" on a diff.

See this PR for an example implementation.

Screen.Recording.2026-08-31.at.11.40.18.PM.mov

[juliusmarminge]

Closing as a duplicate of #6965, which already covers attaching comments on assistant responses to the next prompt. #8744 is a useful implementation reference for that discussion.