Dart 3.9 / Flutter 3.35 [first LLM release] (#2714)

I got carried away with Gemini and basically rewrote CI and the release
process for the new LLM reality. This work was largely completed by
Gemini.

- Bump all SDK versions to the current beta (3.9.0-0)
- Run `flutter channel beta`
- Wrote `ci_script.dart` to replace the bash scripts
- Converted repository to pub workspace #2499 
- Added llm.md and release.md
- Added redirect for deprecated Samples Index

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I have added sample code updates to the [changelog].
- [x] I updated/added relevant documentation (doc comments with `///`).

navigation_and_routing/lib/src/app.dart

@@ -17,8 +17,12 @@ import 'screens/sign_in.dart';
 import 'widgets/book_list.dart';
 import 'widgets/fade_transition_page.dart';
 
-final appShellNavigatorKey = GlobalKey<NavigatorState>(debugLabel: 'app shell');
-final booksNavigatorKey = GlobalKey<NavigatorState>(debugLabel: 'books shell');
+final appShellNavigatorKey = GlobalKey<NavigatorState>(
+  debugLabel: 'app shell',
+);
+final booksNavigatorKey = GlobalKey<NavigatorState>(
+  debugLabel: 'books shell',
+);
 
 class Bookstore extends StatefulWidget {
   const Bookstore({super.key});