Support iTerm2 terminal on Mac with /terminal-setup

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

I want to use - for newlines in my input without CLI interpreting it as input

### Proposed solution

_No response_

### Example prompts or workflows

_No response_

### Additional context

This used to work until a recent build - Shift-Enter gave me a new line in input without sending the command. So it feels like a regression.

Start with the /terminal-setup entry point and reproduce the Shift-Enter behavior in iTerm2 on macOS. Investigate how terminal input is handled there; done means Shift-Enter inserts a newline without submitting the command.

索引モデルが issue の本文から書いたものです。