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.

首先定位 Changes/diff 视图以及渲染评论线程的代码。定义导航范围，然后验证可见的 Previous/Next 控件能够在各线程之间移动，并验证可选的键盘快捷键和位置计数器的行为保持一致。

由索引模型根据 Issue 内容生成。