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/image_page.dart

@@ -19,10 +19,9 @@ class ImagePage extends StatelessWidget {
   DialogRoute _showDialog(BuildContext context) {
     return DialogRoute<void>(
       context: context,
-      builder:
-          (context) => const AlertDialog(
-            title: Text('Image saved! (not really though)'),
-          ),
+      builder: (context) => const AlertDialog(
+        title: Text('Image saved! (not really though)'),
+      ),
     );
   }
 
@@ -49,7 +48,9 @@ class ImagePage extends StatelessWidget {
           ContextMenuRegion(
             contextMenuBuilder: (context, offset) {
               return AdaptiveTextSelectionToolbar.buttonItems(
-                anchors: TextSelectionToolbarAnchors(primaryAnchor: offset),
+                anchors: TextSelectionToolbarAnchors(
+                  primaryAnchor: offset,
+                ),
                 buttonItems: <ContextMenuButtonItem>[
                   ContextMenuButtonItem(
                     onPressed: () {