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/app/useComposerState.ts

@@ -257,13 +257,14 @@ export function useComposerState({
   const openEditor = useCallback(async () => {
     const dir = mkdtempSync(join(tmpdir(), 'hermes-'))
     const file = join(dir, 'prompt.md')
+    const [cmd, ...args] = resolveEditor()
 
     writeFileSync(file, [...inputBuf, input].join('\n'))
 
     let exitCode: null | number = null
 
     await withInkSuspended(async () => {
-      exitCode = spawnSync(resolveEditor(), [file], { stdio: 'inherit' }).status
+      exitCode = spawnSync(cmd!, [...args, file], { stdio: 'inherit' }).status
     })
 
     try {