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 `///`).

testing_app/lib/screens/home.dart

@@ -52,7 +52,8 @@ class ItemTile extends StatelessWidget {
       padding: const EdgeInsets.all(8.0),
       child: ListTile(
         leading: CircleAvatar(
-          backgroundColor: Colors.primaries[itemNo % Colors.primaries.length],
+          backgroundColor:
+              Colors.primaries[itemNo % Colors.primaries.length],
         ),
         title: Text('Item $itemNo', key: Key('text_$itemNo')),
         trailing: IconButton(