Allow users to configure submit key (e.g., Ctrl+Enter) to prevent accidental submission with IME input

[nuitsjp]

Describe the feature or problem you'd like to solve

Accidental prompt submission when using IME (Input Method Editor) for CJK languages, because the Enter key is used both for IME conversion confirmation and prompt submission.

Proposed solution

When using languages that require an IME — such as Japanese, Chinese, and Korean — the Enter key serves double duty: it confirms character conversion in the IME and submits the prompt in Copilot CLI. This leads to frequent accidental submissions and wasted tokens.

Allow users to configure which key submits the prompt and which key inserts a newline. For example:

Setting	Submit	Newline
Default (current)	Enter	Shift+Enter / Ctrl+Enter
IME-friendly	Ctrl+Enter	Enter

This could be a configuration option (e.g., in ~/.config/github-copilot/config.json) or a /settings toggle in the TUI.

The key point is that users should be able to swap the roles of Enter and Ctrl+Enter (or Shift+Enter), not just add alternative newline keys.

Example prompts or workflows

1. A Japanese user types: "このファイルをリファクタリングして". They press Enter to confirm the IME conversion of "して" → the incomplete prompt is accidentally submitted.
2. A user sets submit key to Ctrl+Enter in config → Enter now inserts a newline, Ctrl+Enter submits — matching muscle memory from Slack/Discord.
3. A Korean user composing Hangul hits Enter to finalize syllable blocks → with configurable submit key, they avoid wasting tokens on partial prompts.

Additional context

• Related: New Flag Request: --confirm-send #786 (--confirm-send flag) — addresses accidental submission via double-Enter confirmation. This proposal is complementary.
• Related: SHIFT + ENTER should spawn a line break, but executes the prompt instead. #1481, Ctrl+Enter hotkey not working in TUI (inserts newline instead) #1437, Add Ctrl+J for new line #660, agency copiot cli in VS code issue with SHIFT+ENTER #1871 — existing issues around Enter/Shift+Enter/Ctrl+Enter behavior, all of which assume Enter=submit is fixed.
• This is a common feature in chat applications (Slack, Discord, Telegram all offer this toggle).
• IME-based input affects a large user base: Japanese, Chinese, Korean, and other CJK language users.

[tats-u]

It should be the default out-of-the-box behavior in Japanese (and Traditional Chinese) environments. (Simplified Chinese mainly uses Space to commit a phrase and Korean seldom uses han characters. More feedbacks are welcome)

[tats-u]

Most Japanese who get used to the crappy behavior in chat and messaging applications that Enter without modifier keys will send the input try to press Shift+Enter for line break and Ctrl+Enter for submit. Copilot CLI is a contrarian.