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 `///`).

add_to_app/multiple_flutters/multiple_flutters_module/lib/main.dart

@@ -82,7 +82,10 @@ class _MyHomePageState extends State<MyHomePage> {
               '$_counter',
               style: Theme.of(context).textTheme.headlineMedium,
             ),
-            TextButton(onPressed: _incrementCounter, child: const Text('Add')),
+            TextButton(
+              onPressed: _incrementCounter,
+              child: const Text('Add'),
+            ),
             TextButton(
               onPressed: () {
                 _channel.invokeMethod<void>("next", _counter);