Support batching review comments before submitting to the agent

### Feature summary

_No response_

### What problem are you trying to solve?

When reviewing code changes (diffs), comments can only be submitted one at a time, and each comment is sent to the agent immediately. There's no way to add multiple comments across a diff and submit them together as a single batch.

This means:

- The agent starts reacting before I've finished my full review.
- Reviewing multi-file or multi-hunk changes requires many separate round-trips.
- The agent lacks the full context of my feedback when it begins making fixes.

### Proposed solution

Add a batched review flow, similar to GitHub's PR review experience:

1. Let me add multiple review comments across the diff as I read through it, kept as **pending/draft** (not yet sent).
2. Provide a single **"Submit review"** action that bundles all pending comments and sends them to the agent (e.g. Copilot CLI) at once.
3. The agent then processes the whole batch in one turn, with full context of all my feedback.

### Workflow impact

- Mirrors the familiar GitHub PR review flow (start review → add comments → submit).
- Gives the agent complete context up front, leading to more coherent fixes and fewer redundant iterations.
- Faster, less interruptive review experience for larger change sets.

### Installation context

_No response_

### Additional context

_No response_

Start in the GitHub Copilot app's diff-review experience and trace how individual review comments are currently submitted to the agent. Done means comments can remain pending across multiple files or hunks, a single “Submit review” action sends them together, and the agent processes the batch in one turn.

Written by the indexing model from the issue text.