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 `///`).

context_menus/lib/modified_action_page.dart

@@ -22,10 +22,9 @@ class ModifiedActionPage extends StatelessWidget {
   DialogRoute _showDialog(BuildContext context) {
     return DialogRoute<void>(
       context: context,
-      builder:
-          (context) => const AlertDialog(
-            title: Text('Copied, but also showed this dialog.'),
-          ),
+      builder: (context) => const AlertDialog(
+        title: Text('Copied, but also showed this dialog.'),
+      ),
     );
   }