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/useInputHandlers.ts

@@ -366,10 +366,13 @@ export function useInputHandlers(ctx: InputHandlerContext): InputHandlerResult {
       return voiceRecordToggle()
     }
 
-    // Ctrl+G, plus Alt+G fallback for VSCode/Cursor (they bind Ctrl+G to
-    // "Find Next" before the TUI sees it; Alt+G arrives as meta+g).
+    // Cmd/Ctrl+G, plus Alt+G fallback for VSCode/Cursor (they bind the
+    // primary keystroke to "Find Next" before the TUI sees it; Alt+G
+    // arrives as meta+g across platforms).
     if (ch.toLowerCase() === 'g' && (isAction(key, ch, 'g') || key.meta)) {
-      return cActions.openEditor()
+      return void cActions.openEditor().catch((err: unknown) => {
+        actions.sys(err instanceof Error ? `failed to open editor: ${err.message}` : 'failed to open editor')
+      })
     }
 
     // shift-tab flips yolo without spending a turn (claude-code parity)