[agent-guidance] Name DevTools shared helpers in .gemini/styleguide.md (#10041)

* [agent-guidance] Name DevTools shared helpers in .gemini/styleguide.md

Maintainers repeatedly ask for specific shared widgets and utilities
(CenteredMessage, NotifierSwitch, MultiValueListenableBuilder,
safeUnawaited, Logger, mockConnectedApp, ...). List them under
Code Reuse & DRY so the review bot can suggest them.

* Address review: narrow NotifierSwitch, add theme.dart constants and styles

- Suggest NotifierSwitch only for a raw Switch nested inside a
  ValueListenableBuilder (what NotifierSwitch wraps).
- Point at theme.dart constants and shared text styles instead of raw
  numbers or raw TextStyles.