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 `///`).

animations/test/misc/focus_image_test.dart

@@ -33,7 +33,9 @@ void main() {
       expect(finalSize, greaterThan(initialSize));
     });
 
-    testWidgets('Final inkwell on tap goes back to the grid', (tester) async {
+    testWidgets('Final inkwell on tap goes back to the grid', (
+      tester,
+    ) async {
       await tester.pumpWidget(createFocusImageScreen());
 
       // Tap on the ink well at index 0.