Agents: Add comment to session content as context for the next turn, and let Send fork the session

Problem

In the Agents window, selecting text inside an assistant response offers a single
action: Add Question. Submitting it immediately turns the typed text into a turn
in a session forked from that selection. That action is useful, but it is the only
option: I cannot park a short comment on part of a response and have it carried
along when I send the next turn.

The workarounds are both poor:

• When one response needs several comments, the quoted text and the comments all
  have to be typed into the chat input by hand, which makes the link between a
  comment and its place in the response unclear.
• Use Add Question and continue in the forked session, which splits the
  conversation even when the remark is anchored to quoted text and does not
  deserve a session of its own.

Where the next turn goes is also fixed today: Add Question always forks, and a
normal send always continues the current session. I want to decide which of the
two happens when I send.

Proposed solution

1. When text inside an assistant response is selected, offer Add comment next to
   Add Question, with the same interaction as comments in the Markdown editor:
   type the comment, submit it, and it stays attached to the selected range of the
   response.
2. Carry every comment attached to the current turn into the context of the next
   chat request, so the agent sees both the quoted text and the comment text
   without me restating them.
3. Keep the existing Send button as it is (continue the current session) and add a
   second way to send, where the resulting conversation is a new session forked
   from the current turn. Put it in the Send button's menu. The shortcut is only a
   suggestion: Ctrl+Alt+Enter, or Cmd+Alt+Enter on macOS. I cannot tell whether
   that combination is already taken by an existing command, or whether it matches
   what most developers would expect for this action, so please treat it as a
   proposal.

Alternatives considered

• Keep only Add Question, but also offer a way to attach quoted excerpts to the
  chat input, the way code excerpts are quoted today. This still makes the comment
  list hard to review precisely, because the excerpt index each comment refers to
  has to be declared by hand.
• Always fork when the next turn is sent while comments are present. That default
  is worse, because most comments are small corrections that belong in the current
  conversation.

Additional context

• The Markdown editor comment flow already solves anchoring for files. This
  request asks for the same kind of anchor on assistant responses.
• This request does not replace Add Question; the two actions would sit side by
  side.

Environment

• VS Code: 1.138.0 (x64), stable
• OS: Windows 10 Pro, 10.0.19045, 64-bit
• Area: Agents window, assistant responses and the chat input

Reported by @BlazeNebula. Drafted with GitHub Copilot (agent-assisted).