fix(tui): address Copilot review on editor handoff

- resolveEditor() now returns argv (string[]) so EDITOR='code --wait'
  and VISUAL='emacsclient -t' tokenize correctly into spawnSync's
  separate command + args. Previously the whole string was passed as
  argv[0] and would ENOENT.
- Skip the POSIX X_OK PATH walk on Windows; return ['notepad.exe']
  there since fs.constants.X_OK is not meaningful and PATHEXT-based
  resolution would need its own implementation.
- Surface openEditor() rejections via actions.sys instead of letting
  them become unhandled promise rejections in the useInput callback.
- Hotkey docs/comment now say Cmd/Ctrl+G to match isAction()'s
  platform-action-modifier behavior (Cmd on macOS, Ctrl elsewhere).

ui-tui/src/content/hotkeys.ts

@@ -18,7 +18,7 @@ const copyHotkeys: [string, string][] = isMac
 export const HOTKEYS: [string, string][] = [
   ...copyHotkeys,
   [action + '+D', 'exit'],
-  [action + '+G / Alt+G', 'open $EDITOR for prompt (Alt+G in VSCode/Cursor)'],
+  [action + '+G / Alt+G', 'open $EDITOR (Alt+G fallback for VSCode/Cursor)'],
   [action + '+L', 'new session (clear)'],
   [paste + '+V / /paste', 'paste text; /paste attaches clipboard image'],
   ['Tab', 'apply completion'],