feat: Support additional external editors (micro, ne, helix, etc.)

[efimsky]

Summary

Currently tnc supports a limited set of external text editors. Extend the editor integration to include modern, user-friendly TUI editors that users may prefer over vi/nano.

Motivation

Different users have different editor preferences. Modern TUI editors like micro offer GUI-like keybindings (Ctrl+S, Ctrl+C/V) that are more approachable than modal editors. Supporting a broader list out-of-the-box improves UX without forcing users into config-file editing.

Proposed editors to support

• micro — modern, intuitive, mouse + standard shortcuts
• ne (GNU nice editor) — menu-driven, traditional friendly UX
• helix — modal, LSP built-in, discoverable
• kakoune — selection-first modal editor
• neovim — already common, ensure first-class support
• emacs -nw — terminal Emacs

Acceptance criteria

• Auto-detect installed editors on PATH and offer them as options
• Honor $EDITOR / $VISUAL environment variables as the default
• Allow user to configure a preferred editor in tnc config
• Document supported editors in README
• Fallback chain: $VISUAL → $EDITOR → first detected from known list → vi

Notes

$EDITOR / $VISUAL should take precedence over any built-in detection to respect user environment.