Add /edit <file> command to open files in terminal editors

Currently, Claude Code can't open files in external editors without breaking the session flow. When trying to run vim file.txt via Bash tool, it spawns in a separate process that can't be accessed.

Request: Add a /edit <file> command that:

1. Suspends Claude Code (like Ctrl+Z)
2. Opens the file in a terminal-based editor
3. Resumes Claude Code when editor exits

Implementation notes:

• Should only support terminal editors (vim, nano, emacs, etc.)
• GUI editors like VSCode would break terminal flow
• Check $VISUAL then $EDITOR environment variables
• For GUI editors, existing Bash tool with code file.txt works fine

Use case: Seamlessly edit files during a Claude Code session without having to exit or use separate terminals.

Example:

> /edit ~/.claude/settings.local.json
[Opens in vim/nano/emacs based on $VISUAL or $EDITOR]
[Returns to Claude Code after :wq]

Similar to how git commit opens an editor and returns to the shell.