External Editor Support to compose prompt in Claude Code

Summary

Add support for opening an external editor to compose prompts in Claude Code, similar to how git commit. May be /editor opens the configured $EDITOR to write the prompt.

Problem Statement

When using Claude Code in terminal environments, especially over SSH, users often need to write complex, multi-line prompts. The current inline prompt input has limitations:

• No syntax highlighting or advanced editing features
• Difficult to compose long, structured prompts
• Cannot leverage existing editor configurations (vim/emacs/nano settings)
• Poor experience when working over SSH with limited terminal capabilities
• No easy way to reuse or template prompts from files

Benefits

• Improved productivity: Leverage familiar editor configurations
• Better prompt quality: Easier to write well-structured, detailed prompts
• SSH-friendly: Works well in remote terminal sessions

This feature would significantly improve the Claude Code experience for power users and those working in terminal-heavy environments.