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 `///`).

date_planner/lib/event_list.dart

@@ -36,11 +36,10 @@ class EventList extends StatelessWidget {
                     // Blocked on https://github.com/flutter/flutter/issues/42560.
                     Event? newEvent = await Navigator.of(context).push(
                       CupertinoPageRoute<Event>(
-                        builder:
-                            (_) => EventEditor(
-                              event: Event(title: 'New event'),
-                              isNew: true,
-                            ),
+                        builder: (_) => EventEditor(
+                          event: Event(title: 'New event'),
+                          isNew: true,
+                        ),
                       ),
                     );
 
@@ -69,17 +68,15 @@ class EventList extends StatelessWidget {
                           EventRow(
                             event: e,
                             onTap: () async {
-                              Event? updatedEvent = await Navigator.of(
-                                context,
-                              ).push(
-                                CupertinoPageRoute<Event>(
-                                  builder:
-                                      (_) => EventEditor(
+                              Event? updatedEvent = await Navigator.of(context)
+                                  .push(
+                                    CupertinoPageRoute<Event>(
+                                      builder: (_) => EventEditor(
                                         event: e.copy(),
                                         isNew: false,
                                       ),
-                                ),
-                              );
+                                    ),
+                                  );
                               if (updatedEvent == null) {
                                 // The editor passes back null when it deleted
                                 // the element.