tui: Shift+Enter submits instead of inserting a newline #43222

Summary

In the opencode prompt input, Shift+Enter submits the message instead of
inserting a newline. Since plain Enter also submits, there is currently no
way to enter multi-line prompts, which significantly limits prompt formatting.

Environment

• opencode version: 1.18.18
• OS: Darwin 24.6.0 (darwin arm64)
• Terminal: Ghostty; TERM=xterm-256color; COLORTERM=truecolor
• Shell: /bin/zsh
• Install/channel: Release channel appears likely; exact channel unavailable
• Active plugins: @knikolov/opencode-plugin-simple-memory; no local workspace
  plugin files found

Reproduction

1. Open opencode and focus the prompt input.
2. Type text.
3. Press Shift+Enter.
4. Observe that the message is submitted instead of a newline being inserted.

Expected Behavior

Enter should submit the prompt, while Shift+Enter should insert a newline
without submitting the message.

Actual Behavior

Both Enter and Shift+Enter submit the prompt. There is no available keyboard
shortcut for inserting a newline.

Additional Context

• Reproducible consistently.
• This behavior has existed since opencode was first installed.
• No recent opencode, terminal, configuration, or plugin change is known.
• The issue prevents composing multi-line prompts.