Add a diff-only view/keybind for reviewing just agent code changes

### Describe the feature or problem you'd like to solve

Copilot CLI has useful visibility controls, but none that focus specifically on **code line changes only**.

- `Ctrl+O` / expanded timeline shows too much: shell commands, script output, session-state files, markdown/session artifacts, and other operational noise.
- `Ctrl+T` is about reasoning, not code review.
- `/diff` is helpful, but it doesn't map to the quick "show me exactly what code changed from the agent" interaction I want while staying inside the main terminal flow.

When I'm working in the terminal, I care most about **how the agent changed my code**. I usually don't care about session-state updates, helper scripts, or verbose command output.

### Proposed solution

Add a **diff-only review mode** in Copilot CLI for agent edits.

Ideal behavior:

1. A dedicated keybind to toggle **code diff view only**.
2. Show only edited file hunks, grouped by file.
3. Hide shell/script output, session artifacts, state files, and unrelated timeline items by default.
4. Support scoping such as:
- **last turn only**
- **all changes in current session**
5. Render diffs in a clean terminal-native viewer instead of requiring users to expand the entire timeline or open an external editor.

This would make it much easier to audit agent changes quickly and confidently from inside the CLI.

### Example prompts or workflows

1. I ask Copilot CLI to refactor a function, then hit a keybind to see only the code hunks it changed in that turn.
2. I let the agent make several edits, then toggle a session-wide diff-only view without seeing all shell commands and background task output.
3. I review changes in a terminal-only workflow without switching to VS Code or manually running `git diff`.
4. I compare the agent's edits before deciding whether to keep working, rewind, or undo.

### Additional context

The main gap is not "can I see changes at all?" — it's that the current visibility options are too broad or noisy for the common "review only code edits" workflow.

A dedicated diff-only view would be especially valuable for users who prefer terminal-first workflows and want a fast audit surface similar to focused code-diff review in other coding agents.

首先检查现有的 Ctrl+O 展开时间线、Ctrl+T 推理视图和 /diff 工作流，以了解它们当前的作用域和渲染方式。定义终端原生的仅 diff 模式如何识别代码 hunk、分组已编辑文件，以及区分上一轮更改与会话中的所有更改。当交互、键绑定、作用域行为以及对非代码工件的处理都已明确到足以实现和审查时，即视为完成。

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