Dart 3.9 / Flutter 3.35

[ericwindmill]

I got carried away with Gemini and basically rewrote CI and the release process for the new LLM reality.

• 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
• Added llm.md and release.md
• Added redirect for deprecated Samples Index

Pre-launch Checklist

• I read the [Flutter Style Guide] recently, and have followed its advice.
• I signed the [CLA].
• I read the [Contributors Guide].
• I have added sample code updates to the [changelog].
• I updated/added relevant documentation (doc comments with ///).