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 `///`).

code_sharing/server/bin/server.dart

@@ -9,10 +9,9 @@ import 'package:shelf_router/shelf_router.dart';
 int count = 0;
 
 // Configure routes.
-final _router =
-    Router()
-      ..post('/', _incrementHandler)
-      ..get('/', _getValueHandler);
+final _router = Router()
+  ..post('/', _incrementHandler)
+  ..get('/', _getValueHandler);
 
 Future<Response> _incrementHandler(Request request) async {
   final incr = Increment.fromJson(json.decode(await request.readAsString()));