[FEATURE] Support shift+enter via common control sequence identifiers

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Many users have issues with the "shift+enter" support because Claude Code currently relies on a terminal-level remapping of that input to instead provide an explicit newline character. This is necessary for the program to distinguish the remapped input from a normal enter keypress.

This requires the whole terminal-setup command, causes behavior that changes depending on which terminal you are running within, and the feature can be broken based on the user's modification of their own config files for other programs.

Proposed Solution

In other sophisticated TUI applications, distinguishing these inputs can be done with control sequence identifiers. This would greatly ease user onboarding, as many modern terminals are setup to use one of these two codes. (As an example, in a new terminal window run cat with no input and then press shift+enter to show the exact sequence identifier received over stdin)

The two codes that could support this ought to be:

1. CSI u code for shift+enter: ^[[13;2u
2. modifyOtherKeys code for shift+enter: ^[[27;2;13~

CSI u is from the extended keys format, is more widely supported and more modern, and would be a great starting point!

Alternative Solutions

_No response_

Priority

Medium - Would be very helpful

Feature Category

Interactive mode (TUI)

Use Case Example

_No response_

Additional Context

Attached is a screenshot of CSI u code for shift+enter on ghostty within Ubuntu.

<img width="172" height="140" alt="Image" src="https://github.com/user-attachments/assets/146cb0bd-3500-4fc9-befd-12364efd9eec" />

Additionally, more details can be found in my exploration within OpenCode: https://github.com/sst/opencode/issues/1505#issuecomment-3411334883