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 `///`).

analysis_defaults/.gitignore

@@ -1,3 +1,4 @@
 # https://dart.dev/guides/libraries/private-files
 # Created by `dart pub`
 .dart_tool/
+.build/

analysis_defaults/lib/flutter.yaml

@@ -1,5 +1,9 @@
 include: package:flutter_lints/flutter.yaml
 
+formatter:
+  trailing_commas: preserve
+  page_width: 79
+
 analyzer:
   language:
     strict-casts: true

analysis_defaults/pubspec.yaml

@@ -1,11 +1,14 @@
 name: analysis_defaults
 description: Analysis defaults for flutter/samples
 publish_to: none
+resolution: workspace
 
 environment:
-  sdk: ^3.7.0-0
+  sdk: ^3.9.0-0
 
 # NOTE: Code is not allowed in this package. Do not add more dependencies.
 # The `flutter_lints` dependency is required for `lib/flutter.yaml`.
 dependencies:
-  flutter_lints: ^5.0.0
+  flutter_lints: ^6.0.0
+
+