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/ci_script.dart

@@ -0,0 +1,67 @@
+import 'dart:io';
+import 'package:path/path.dart' as path;
+import 'package:yaml/yaml.dart';
+
+Future<void> main() async {
+  final rootDir = Directory.current;
+  final pubspecFile = File(path.join(rootDir.path, 'pubspec.yaml'));
+  final pubspecContent = await pubspecFile.readAsString();
+  final pubspecYaml = loadYaml(pubspecContent);
+
+  final workspace = pubspecYaml['workspace'] as YamlList?;
+  if (workspace == null) {
+    print('No workspace found in pubspec.yaml');
+    exit(1);
+  }
+
+  // pub workspace, only run 'get' once
+  await _runCommand('flutter', ['pub', 'get'], workingDirectory: rootDir.path);
+
+  final packages = workspace.map((e) => e.toString()).toList();
+
+  for (final package in packages) {
+    final packagePath = path.join(rootDir.path, package);
+    print('== Testing \'$package\' ==');
+    await _runCommand('dart', [
+      'analyze',
+      '--fatal-infos',
+      '--fatal-warnings',
+    ], workingDirectory: packagePath);
+
+    await _runCommand('dart', ['format', '.'], workingDirectory: packagePath);
+
+    if (await Directory(path.join(packagePath, 'test')).exists()) {
+      final packagePubspecFile = File(path.join(packagePath, 'pubspec.yaml'));
+      final packagePubspecContent = await packagePubspecFile.readAsString();
+      if (packagePubspecContent.contains('flutter:')) {
+        await _runCommand('flutter', [
+          'test',
+          '--no-pub',
+        ], workingDirectory: packagePath);
+      } else {
+        await _runCommand('dart', ['test'], workingDirectory: packagePath);
+      }
+    }
+  }
+}
+
+Future<void> _runCommand(
+  String executable,
+  List<String> arguments, {
+  String? workingDirectory,
+}) async {
+  final process = await Process.start(
+    executable,
+    arguments,
+    workingDirectory: workingDirectory,
+    runInShell: true,
+    mode: ProcessStartMode.inheritStdio,
+  );
+  final exitCode = await process.exitCode;
+  if (exitCode != 0) {
+    print(
+      'Command "$executable ${arguments.join(' ')}" failed with exit code $exitCode in $workingDirectory',
+    );
+    exit(exitCode);
+  }
+}