feat(shell): support Shift+Enter for inserting newlines

Feature Request: Support Shift+Enter for inserting newlines

What feature would you like to see?

Add Shift+Enter as an alternative shortcut for inserting a newline in the interactive prompt, alongside the existing Ctrl-J and Alt-Enter bindings.

Current behavior

Shortcut	Action
Enter	Submit message
Ctrl-J	Insert newline
Alt-Enter	Insert newline
Shift+Enter	Not supported

Desired behavior

Shortcut	Action
Enter	Submit message
Shift+Enter	Insert newline
Ctrl-J / Alt-Enter	Keep for backward compatibility

Why this matters

I am an active user of multiple AI coding agents. My daily workflow includes Claude Code, Codex CLI, OpenCode, and Amp. In all of them, the pattern is consistent:

• Enter → submit
• Shift+Enter → insert newline

This has become the de-facto standard across modern chat UIs (ChatGPT, Claude, Discord, Slack, Cursor, VS Code Copilot Chat, etc.). Muscle memory from these platforms makes the current Ctrl-J / Alt-Enter behavior in Kimi CLI feel unintuitive and frustrating.

I have lived in China for twelve years. I am genuinely proud to see Chinese AI companies like Moonshot building world-class models and tools. Kimi K2.5 and K2 are disruptive and impressive. I want to use Kimi Code CLI more — it is already quite good for many tasks, and I believe it has enormous potential. But this small UX friction constantly pulls me out of flow. Every time I instinctively hit Shift+Enter and the message submits prematurely, I have to stop and remind myself: "Kimi is different."

I do not believe I am the only one who experiences this. Supporting Shift+Enter would align Kimi CLI with the expectations of millions of developers who are trained on this standard. It would be a small change with a large community impact.

Proposed implementation

The change should be minimal and non-breaking:

1. Add Shift+Enter (s-enter in prompt_toolkit) alongside the existing newline keybindings in src/kimi_cli/ui/shell/prompt.py.
2. Update the bottom toolbar hint from ctrl-j: newline to shift-enter / ctrl-j: newline.
3. Update the English documentation (docs/en/reference/keyboard.md, docs/en/guides/interaction.md) to list Shift+Enter.

No terminal protocol manipulation, no breaking changes to existing behavior. Ctrl-J and Alt-Enter should continue to work exactly as they do today.

Environment

• kimi-cli version: latest (1.43.0)
• OS: macOS / Linux / Windows (applies to all platforms)

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功能请求：支持 Shift+Enter 插入换行

期望的功能

在交互式提示符中增加 Shift+Enter 作为插入换行的快捷键，与现有的 Ctrl-J 和 Alt-Enter 并存。

当前行为

快捷键	动作
Enter	发送消息
Ctrl-J	插入换行
Alt-Enter	插入换行
Shift+Enter	不支持

期望的行为

快捷键	动作
Enter	发送消息
Shift+Enter	插入换行
Ctrl-J / Alt-Enter	保留，向后兼容

为什么这很重要

我是多个 AI 编程代理的活跃用户。日常工作中我使用 Claude Code、Codex CLI、OpenCode 和 Amp。它们都有一个一致的操作模式：

• Enter → 发送
• Shift+Enter → 插入换行

这已成为现代聊天界面的事实标准（ChatGPT、Claude、Discord、Slack、Cursor、VS Code Copilot Chat 等）。来自这些平台的肌肉记忆使得 Kimi CLI 当前的 Ctrl-J / Alt-Enter 行为显得不直观且令人沮丧。

我在中国生活了十二年。看到 Moonshot 这样的中国 AI 公司正在构建世界一流的模型和工具，我感到非常自豪。Kimi K2.5 和 K2 具有颠覆性且令人印象深刻。我希望更多地使用 Kimi Code CLI——它在很多任务上已经相当出色，我相信它有巨大的潜力。但这个小小的用户体验摩擦总是让我中断心流。每次我本能地按下 Shift+Enter 而消息却提前发送时，我都不得不停下来提醒自己："Kimi 不一样。"

我相信不止我一个人遇到这个问题。支持 Shift+Enter 将使 Kimi CLI 与数百万已经熟悉这一标准的开发者的期望保持一致。这将是一个改动很小、但社区影响很大的改进。

建议的实现方式

改动应该最小化且不破坏现有功能：

1. 在 src/kimi_cli/ui/shell/prompt.py 中，将 Shift+Enter（prompt_toolkit 中为 s-enter）添加到现有的换行快捷键绑定中。
2. 将底部工具栏提示从 ctrl-j: newline 更新为 shift-enter / ctrl-j: newline。
3. 更新英文文档（docs/en/reference/keyboard.md、docs/en/guides/interaction.md）以列出 Shift+Enter。

不需要操作终端协议，也不会破坏现有行为。Ctrl-J 和 Alt-Enter 应继续完全按今天的方式工作。

Start in src/kimi_cli/ui/shell/prompt.py by reviewing the existing Ctrl-J and Alt-Enter newline bindings and toolbar hint. Update the English entries in docs/en/reference/keyboard.md and docs/en/guides/interaction.md. Done means Shift+Enter inserts a newline while Enter still submits and the existing bindings continue to work.