Plan editor should respect $EDITOR environment variable

Summary

When editing a plan in Claude Code, the built-in plan editor is used rather than the user's configured $EDITOR environment variable.

Expected behavior

Claude Code should respect $EDITOR (or $VISUAL) and open plan files in the user's preferred editor (e.g., nvim, vim, code).

Current behavior

The plan editor uses its own built-in editing experience, ignoring the user's shell environment configuration.