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 `///`).

tool/pubspec.yaml

@@ -0,0 +1,13 @@
+name: repo_tool
+description: A tool to automate release cleanup for the samples monorepo.
+resolution: workspace
+
+environment:
+  sdk: ^3.9.0-0
+
+dependencies:
+  args: ^2.4.2
+  io: ^1.0.4
+  path: ^1.9.0
+  yaml: ^3.1.2
+  yaml_edit: ^2.1.1