Add next/previous navigation for comments in Changes/Diff view

## Problem

When reviewing a session's Changes (diff) view and leaving comments — including unresolved comments and back-and-forth conversations with the LLM — there's no quick way to jump between them. I end up scrolling up and down the diff hunting for my open threads, which gets painful on larger changes.

## Proposed feature

Add explicit **Previous / Next comment** navigation in the Changes / diff UI. Ideally:

- Visible UX affordance (e.g., up/down arrow buttons in a small floating control, or buttons in the file header) — there appears to be room for it.
- Jumps between all comment threads in the current diff (or scoped to unresolved / open threads).
- Keyboard shortcuts as well (e.g., `j`/`k` or `n`/`p`). If these already exist, they're not discoverable — surfacing them in the UI would help.
- Bonus: a small counter like `2 of 7` so you know where you are in the list of threads.

## Why it matters

- Reviewing my own comments and the LLM's responses is a core part of the iteration loop.
- Scrolling to find threads breaks flow, especially when a session has produced a large diff.
- Discoverable Prev/Next controls make the review surface feel much more like a real code review tool.

## Notes

Filed via Copilot CLI on behalf of @cirvine-MSFT.

Bắt đầu bằng cách xác định chế độ xem Changes/diff và đoạn mã kết xuất các luồng bình luận. Xác định phạm vi điều hướng, sau đó kiểm tra rằng các điều khiển Previous/Next hiển thị cho phép di chuyển qua các luồng và rằng các phím tắt tùy chọn cùng bộ đếm vị trí hoạt động nhất quán.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.