Support Shift+Enter for multiline input (industry standard)

Currently Claude Code uses Enter for submitting and requires Alt+Enter for newlines.
This goes against the industry standard where virtually all modern tools use:

• Shift+Enter for newlines
• Enter for submit

Tools that use Shift+Enter for multiline:

• ChatGPT, GitHub Copilot Chat, VSCode, Slack, Discord, Jupyter, etc.

Proposed solution:

• Make Shift+Enter the default for newlines
• Keep Enter for submit
• Optionally provide /terminal-setup command that works on Linux terminals

This would improve UX consistency and reduce context switching friction.

Environment:

• OS: Linux
• Terminal: Terminator
• Claude Code version: 1.0.2