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 `///`).

code_sharing/shared/pubspec.yaml

@@ -1,15 +1,18 @@
 name: shared
 description: Common data models required by our client and server
 version: 1.0.0
+resolution: workspace
 
 environment:
-  sdk: ^3.7.0-0
+  sdk: ^3.9.0-0
 
 dependencies:
   freezed_annotation: ">=2.1.0 <4.0.0"
   json_annotation: ^4.7.0
 
 dev_dependencies:
+  analysis_defaults:
+    path: ../../analysis_defaults
   build_runner: ^2.2.1
   freezed: ">=2.1.1 <4.0.0"
   json_serializable: ^6.4.0